diff --git a/mkdocs.yml b/mkdocs.yml
index 28ba5d5b5a..c7d4f36f81 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -46,6 +46,7 @@ nav:
           - Quantitative MRI: appendices/qmri.md
           - Arterial Spin Labeling: appendices/arterial-spin-labeling.md
           - Cross modality correspondence: appendices/cross-modality-correspondence.md
+          - Phenotypic data guidelines: appendices/phenotype.md
       - Changelog: CHANGES.md
   - The BIDS Starter Kit:
       - Website: https://bids.neuroimaging.io/getting_started/
diff --git a/src/appendices/phenotype.md b/src/appendices/phenotype.md
new file mode 100644
index 0000000000..e67a169060
--- /dev/null
+++ b/src/appendices/phenotype.md
@@ -0,0 +1,424 @@
+# Tabular phenotypic data guidelines
+
+This appendix is a collection of guidelines and examples
+for curating well-organized tabular phenotypic data.
+
+## Guidelines
+
+These guidelines are intended to improve the organization and clarity of
+tabular phenotypic data like the participants file, sessions file,
+and phenotypic and assessment data.
+
+They are recommendations and are by default ignored during validation.
+You can make them mandatory during validation by setting the
+[`AdditionalValidation` key](../modality-agnostic-files/dataset-description.md#additional-validation)
+to `"Phenotype"` in the `dataset_description.json`.
+
+### 1. Aggregate data across sessions
+
+Aggregate participant information across all sessions into one tabular TSV file per
+measurement or phenotypic assessment and store this file in the `/phenotype` directory.
+Demographic information is a special case and  MUST be aggregated
+in the `participants.tsv` file at the root level of the dataset.
+It is RECOMMENDED to use the `age` column in the `participants.tsv` file
+to record participant age at every session in longitudinal or multi-session data sets.
+
+### 2. Always pair tabular data with data dictionaries
+
+Tabular phenotypic data MUST be prepared as one pair of a tabular file
+in tab-separated value (TSV) format and a corresponding data dictionary
+in JavaScript Object Notation (JSON) format.
+See the [Tabular files section](../common-principles.md#tabular-files) for more information.
+
+### 3. Add `MeasurementToolMetadata` to each tabular phenotypic measurement tool
+
+Whenever possible, it is RECOMMENDED to add `MeasurementToolMetadata` to
+each `phenotype/<measurement_tool_name>.json` data dictionary.
+This improves reusability and provides clarity about the measurement tool.
+See [`MeasurementToolMetadata` in the glossary](../glossary.md#measurementtoolmetadata-metadata) for more.
+
+### 4. Ensure minimal annotation for phenotypic and assessment data
+
+In phenotypic and assessment data each measurement tool SHOULD have an independent
+aggregated data TSV file in which the user collects all subjects, sessions,
+and/or runs of data as one entry per row (with a row defined by
+the smallest unit of acquisition). In other words:
+
+-   Each row MUST start with `participant_id`.
+
+-   Each TSV file MUST contain a `session_id` column when
+    multiple [sessions](../glossary.md#session-entities)[<sup>1</sup>](#footnotes) are present
+    in the data set regardless of whether those sessions are in
+    the `phenotype/` data, `sub-<label>/` data, or a combination of the two.
+
+-   If more than one of the same measurement tool is acquired within
+    the same `session_id`, a `run_id` column MUST be added.
+
+-   Encoding  the acquisition time for a measurement tool’s `session_id`,
+     is RECOMMENDED. This information MUST be stored in the `sessions.tsv`
+     file at the root level of the dataset in the `acq_time` column.
+
+<!-- This block generates a columns table.
+The definitions of these fields can be found in
+  src/schema/rules/tabular_data/*.yaml
+and a guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_columns_table("modality_agnostic.Phenotypes") }}
+
+This rule can be considered as "**if anyone uses sessions, everyone uses sessions**."
+
+### 5. Store demographic data in `participants.tsv` and instrument data in the `/phenotype` directory
+
+The `participants.tsv` file is for demographic information about the participant,
+including longitudinal information such as `age`.
+The `/phenotype` directory is for phenotypic information collected about
+the participants, such as questionnaires, cognitive assessments or tasks.
+Create one tabular `.tsv` file for each instrument or assessment in the `/phenotype` directory.
+
+### 6. Use the sessions file at the root-level
+
+If there is more than one session for any one participant, then
+it is RECOMMENDED to provide a sessions file at the dataset root.
+The sessions file MUST list all sessions for all subjects across
+imaging and tabular phenotypic data.
+If a sessions file is provided, then it MUST begin with a `participant_id` column
+followed immediately by a `session_id` column. The data dictionary JSON file’s
+`session_id` field MUST include `Levels` with the description of each `session_id`.
+
+### 7. Record participant properties in the participants file and session properties in the sessions file
+
+Since the same `participant_id` and `session_id` columns can be used
+similarly in the participants file and the sessions file,
+use the two different files to instead differentiate
+properties of participants versus sessions.
+Properties of participants MAY include things like
+age, sex, race, or household income.
+Properties of sessions MAY include things like
+acquisition time, measurement device properties,
+and indoor or outdoor experimental conditions.
+
+### 8. Use either root-level sessions file or participant-level sessions files, but not both
+
+When you use a sessions file at the dataset-level,
+you MUST NOT provide additional sessions files at the participant-level
+as this might conflict with the inheritance principle.
+
+### 9. Record acquisition time of all sessions with `acq_time`
+
+It is RECOMMENDED to store acquisition time[<sup>2</sup>](#footnotes)
+for tabular phenotypic data in the sessions file in a column named `acq_time`.
+This is consistent with how acquisition time is recorded for MRI data
+and other time-sensitive measurements (for example systolic blood pressure).
+
+### 10. Respect participant privacy when recording acquisition times
+
+When needed to preserve participant privacy, you SHOULD record
+relative acquisition times with respect to the earliest session.
+Relative session acquisition times MAY be listed as durations from
+the earliest session (baseline) in days, months, or years
+using the `acq_time` column.
+
+## Summary
+
+This appendix described guidelines for best tabular phenotypic data.
+In summary, it is RECOMMENDED to always use the participants file
+and separate files by assessment in the `/phenotype/` directory,
+since they each collect different information.
+If you use sessions, then the sessions file is also RECOMMENDED.
+
+## Examples
+
+What follows are a few common use case examples for tabular phenotypic files.
+
+### 1 participant session with both non-tabular and tabular phenotypic data
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "phenotype": {
+      "measurement_tool.json": "",
+      "measurement_tool.tsv": "",
+      },
+   "sub-01": {
+      "anat": {
+         "sub-01_T1w.json": "",
+         "sub-01_T1w.nii.gz": "",
+         }
+      }
+   }
+) }}
+
+Contents of `phenotype/measurement_tool.tsv`
+
+```tsv
+participant_id	measurement_1	measurement_2
+sub-01	value1	value2
+```
+
+### 1 participant with 2 sessions, where 1 session is only tabular phenotype and the other is only imaging
+
+With only one imaging and one phenotypic session each in this example you might want
+to merge both imaging and phenotypic data under one session. But it is more correct to
+have separate sessions for the imaging and phenotypic data, especially if
+the sessions were collected days, weeks, or months apart. You can denote both sessions
+and their acquisition time in the `sessions.tsv` file and have `session_id` `Levels` noted
+in the `sessions.json` sidecar. Below are a CORRECT and an INCORRECT example
+of prepared data following these guidelines.
+
+#### CORRECT
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "sessions.json": "",
+   "sessions.tsv": "",
+   "phenotype": {
+      "measurement_tool.json": "",
+      "measurement_tool.tsv": "",
+      },
+   "sub-01": {
+      "ses-MRI": {
+         "anat": {
+            "sub-01_ses-MRI_T1w.json": "",
+            "sub-01_ses-MRI_T1w.nii.gz": "",
+            }
+         }
+      }
+   }
+) }}
+
+Contents of `sessions.tsv`
+
+```tsv
+participant_id	session_id	acq_time
+sub-01	ses-pheno	2001-01-01T12:05:00
+sub-01	ses-MRI	2001-03-01T13:14:00
+```
+
+Contents of `phenotype/measurement_tool.tsv`
+
+```tsv
+participant_id	session_id	measurement_1	measurement_2
+sub-01	ses-pheno	value1	value2
+```
+
+#### INCORRECT
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "phenotype": {
+      "measurement_tool.json": "",
+      "measurement_tool.tsv": "",
+      },
+   "sub-01": {
+      "anat": {
+         "sub-01_T1w.json": "",
+         "sub-01_T1w.nii.gz": "",
+         }
+      }
+   }
+) }}
+
+Contents of `phenotype/measurement_tool.tsv`
+
+```tsv
+participant_id	measurement_1	measurement_2
+sub-01	value1	value2
+```
+
+A session directory **MUST** be present in the participant directory and
+the `session_id` column **MUST** be present in `phenotype/measurement_tool.tsv` as well.
+Sessions must be used consistently for the combination of tabular and
+non-tabular phenotypic data.
+
+### 2 participants with a mix of tabular phenotypic data and imaging sessions
+
+In this example, participants acquired both
+a phenotypic measurement tool and an MRI during `ses-MRI1`.
+`sub-01` has a `ses-MRI2` with no phenotypic measurement tool acquired
+and `sub-02` has a `ses-pheno` where no MRI was acquired.
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "sessions.json": "",
+   "sessions.tsv": "",
+   "phenotype": {
+      "measurement_tool.json": "",
+      "measurement_tool.tsv": "",
+      },
+   "sub-01": {
+      "ses-MRI1": {
+         "anat": {
+            "sub-01_ses-MRI1_T1w.json": "",
+            "sub-01_ses-MRI1_T1w.nii.gz": "",
+            }
+         },
+      "ses-MRI2": {
+         "anat": {
+            "sub-01_ses-MRI2_T1w.json": "",
+            "sub-01_ses-MRI2_T1w.nii.gz": "",
+            }
+         }
+      },
+   "sub-02": {
+      "ses-MRI1": {
+         "anat": {
+            "sub-02_ses-MRI1_T1w.json": "",
+            "sub-02_ses-MRI1_T1w.nii.gz": "",
+            }
+         }
+      }
+   }
+) }}
+
+Contents of `sessions.tsv`
+
+```tsv
+participant_id	session_id	acq_time
+sub-01	ses-MRI1	2001-01-01T11:12:00
+sub-01	ses-MRI2	2001-07-01T13:14:00
+sub-02	ses-MRI1	2001-01-181T15:16:00
+sub-02	ses-pheno	2001-02-20T12:05:00
+```
+
+Contents of `phenotype/measurement_tool.tsv`
+
+```tsv
+participant_id	session_id	measurement_1	measurement_2
+sub-01	ses-MRI1	value1	value2
+sub-02	ses-MRI1	value3	value4
+sub-02	ses-pheno	value5	value6
+```
+
+### 3 participants with 3 different kinds of sessions among them
+
+The `ses-baseline` session collects an MRI and tabular phenotypic data.
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "participants.json": "",
+   "participants.tsv": "",
+   "sessions.json": "",
+   "sessions.tsv": "",
+   "phenotype": {
+      "survey.json": "",
+      "survey.tsv": "",
+      },
+   "sub-01": {
+      "ses-baseline/": "",
+      "ses-followupMRI/": "",
+      },
+   "sub-02": {
+      "ses-baseline/": "",
+      },
+   "sub-03": {
+      "ses-baseline/": "",
+      "ses-followupMRI/": "",
+      }
+   }
+) }}
+
+Contents of `participants.tsv`. Participant properties that can change
+from session to session belong here especially.
+
+```tsv
+participant_id	session_id	sex	age	gender	race	household_income
+sub-01	ses-baseline	M	10	3	4	5
+sub-01	ses-followupMRI	M	10	3	4	5
+sub-01	ses-interview	M	11	4	4	6
+sub-02	ses-baseline	F	9	1	3	3
+sub-02	ses-interview	F	10	1	7	3
+sub-03	ses-baseline	F	11	2	10	4
+sub-03	ses-followupMRI	F	12	5	10	4
+```
+
+Contents of `sessions.tsv`.
+
+```tsv
+participant_id	session_id	acq_time
+sub-01	ses-baseline	2001-01-01T12:05:00
+sub-01	ses-followupMRI	2001-07-01T13:33:00
+sub-01	ses-interview	2002-01-01T11:21:00
+sub-02	ses-baseline	2001-04-01T11:01:00
+sub-02	ses-interview	2002-04-01T14:08:00
+sub-03	ses-baseline	2001-09-01T11:45:00
+sub-03	ses-followupMRI	2002-03-01T12:17:00
+```
+
+Contents of `sessions.json`. Note how the `session_id` `Levels` are clearly described.
+
+```json
+{
+    "participant_id": {
+        "Description": "BIDS participant identifier"
+    },
+    "session_id": {
+        "Description": "BIDS session identifier",
+        "Levels": {
+            "ses-baseline": "Baseline visit for MRI and assessments",
+            "ses-followupMRI": "6-months after baseline MRI follow-up",
+            "ses-interview": "1-year after baseline in-person follow-up"
+        }
+    },
+    "acq_time": {
+        "Description": "When the data acquisition started"
+    }
+}
+```
+
+Contents of `phenotype/survey.tsv`. Note how `sub-03` does not have
+a row for `ses-interview` because that session was not collected
+and is absent above in the `participants.tsv` and `sessions.tsv` files.
+
+```tsv
+participant_id	session_id	question_1	question_2	question_3
+sub-01	ses-baseline	A	2	no
+sub-01	ses-interview	A	3	yes
+sub-02	ses-baseline	A	2	no
+sub-02	ses-interview	B	1	unsure
+sub-03	ses-baseline	B	3	no
+```
+
+For more complete examples, see the `pheno00*`
+[bids-examples on GitHub](https://github.com/bids-standard/bids-examples/).
+
+## Footnotes
+
+<sup>1</sup> A session is any logical grouping of imaging and behavioral data consistent
+across participants. Session can (but doesn't have to) be synonymous to a visit
+in a longitudinal study. In situations where different data types are obtained over
+several visits (for example fMRI on one day followed by DWI the day after)
+those can still be grouped in one session. Refer to the
+[definition of session](../glossary.md#session-entities) for more details.
+
+<sup>2</sup> Datetime format and the anonymization procedure are
+described in [Units](../common-principles.md#units).
diff --git a/src/common-principles.md b/src/common-principles.md
index c3a395c600..e9948b0b98 100644
--- a/src/common-principles.md
+++ b/src/common-principles.md
@@ -474,7 +474,7 @@ NIfTI header.
 
 ### Tabular files
 
-Tabular data MUST be saved as plain-text, tab-delimited values (TSV) files
+Tabular data MUST be saved as plain-text, tab-separated values (TSV) files
 (with [extension `.tsv`](glossary.md#tsv-extensions)),
 that is, [CSV files](https://en.wikipedia.org/wiki/Comma-separated_values) where commas are replaced by tab characters.
 Tabs MUST be true tab characters and MUST NOT be a series of space characters.
@@ -525,7 +525,7 @@ onset	duration	response_time	trial_type	trial_extra
     are not part of the tabular data file's content.
 
 Tabular files MAY be optionally accompanied by a simple data dictionary
-in the form of a JSON [object](https://www.json.org/json-en.html)
+in the form of a [JSON object](https://www.json.org/json-en.html)
 within a JSON file.
 The JSON files containing the data dictionaries MUST have the same name as
 their corresponding tabular files but with `.json` extensions.
diff --git a/src/modality-agnostic-files/data-summary-files.md b/src/modality-agnostic-files/data-summary-files.md
index d6b5032c2b..96c69b910b 100644
--- a/src/modality-agnostic-files/data-summary-files.md
+++ b/src/modality-agnostic-files/data-summary-files.md
@@ -53,6 +53,9 @@ to date of birth.
 
 ```JSON
 {
+    "participant_id": {
+        "Description": "participant identifier"
+    },
     "age": {
         "Description": "age of the participant",
         "Units": "year"
@@ -81,6 +84,14 @@ to date of birth.
 }
 ```
 
+It is RECOMMENDED to use the `age` column to record participant age
+at every session in longitudinal or multi-session data sets.
+This reduces data duplication across tabular data files. The `Units` of `age`
+do not have to be years so long as the units of the age
+are written in `participants.json`.
+Consider participant privacy or study objectives when selecting
+the `Units` of `age` or the accuracy of `age` data.
+
 ## Samples file
 
 Template:
@@ -199,21 +210,14 @@ meg/sub-control01_task-rest_split-02_meg.nii.gz	1877-06-15T12:15:27
 
 ## Sessions file
 
-Template:
-
-```Text
-sub-<label>/
-    sub-<label>_sessions.tsv
-```
-
-Optional: Yes
-
-In case of multiple sessions there is an option of adding additional
-`sessions.tsv` files describing variables changing between sessions.
-In such case one file per participant SHOULD be added.
-These files MUST include a `session_id` column and describe each session by one and only one row.
-Column names in `sessions.tsv` files MUST be different from group level participant key column names in the
-[`participants.tsv` file](./data-summary-files.md#participants-file).
+In case of multiple sessions there is an option of adding an additional
+`sessions.tsv` file describing each session and variables changing between sessions.
+It is RECOMMENDED to provide this as a single file at the root-level of the dataset.
+It is OPTIONAL to provide these as separate files at the subject-level of the dataset.
+The intent of the sessions file is to describe the sessions
+in a data set and non-demographic variables changing between sessions.
+Column names in `sessions.tsv` files MUST be different from participant key column names in
+the [participants file](#participants-file).
 
 <!-- This block generates a columns table.
 The definitions of these fields can be found in
@@ -223,7 +227,82 @@ and a guide for using macros can be found at
 -->
 {{ MACROS___make_columns_table("modality_agnostic.Sessions") }}
 
-`_sessions.tsv` example:
+`sessions.json` example:
+
+```JSON
+{
+    "participant_id": {
+        "Description": "Participant identifier"
+    },
+    "session_id": {
+        "Description": "Session identifier for the session",
+        "Levels": {
+            "ses-predrug": "session before drug administration",
+            "ses-postdrug": "session after drug administration",
+            "ses-followup": "follow-up session"
+        }
+    },
+    "acq_time": {
+        "Description": "Acquisition time of the session"
+    },
+    "systolic_blood_pressure": {
+        "Description": "Systolic blood pressure measured at the beginning of the session in mmHg"
+    }
+}
+```
+
+### RECOMMENDED: Root-level sessions file
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "sessions.tsv": "",
+   "[sessions.json]": "",
+   }
+) }}
+
+Optional: Yes
+
+An aggregated sessions file is RECOMMENDED to be provided at the dataset root.
+If a root-level sessions file is provided, then it MUST begin with
+a `participant_id` column followed immediately after by a `session_id` column.
+
+`sessions.tsv` example:
+
+```tsv
+participant_id	session_id	acq_time	systolic_blood_pressure
+sub-01	ses-predrug	2009-06-15T13:45:30	120
+sub-01	ses-postdrug	2009-06-16T13:45:30	100
+sub-01	ses-followup	2009-06-17T13:45:30	110
+sub-02	ses-predrug	2009-06-22T12:22:05	105
+sub-02	ses-postdrug	2009-06-23T12:22:05	95
+sub-03	ses-postdrug	2009-06-30T14:06:40	115
+sub-03	ses-followup	2009-07-01T14:06:40	120
+```
+
+### OPTIONAL: Participant-level sessions files
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "sub-<label>": {
+      "sub-<label>_sessions.tsv": "",
+      "[sub-<label>_sessions.json]": "",
+      }
+   }
+) }}
+
+Optional: Yes
+
+When one sessions file per participant is used,
+these files MUST include a `session_id` column and describe each session by one and only one row.
+`sub-<label>/sub-<label>_sessions.tsv` example:
 
 ```tsv
 session_id	acq_time	systolic_blood_pressure
@@ -231,3 +310,36 @@ ses-predrug	2009-06-15T13:45:30	120
 ses-postdrug	2009-06-16T13:45:30	100
 ses-followup	2009-06-17T13:45:30	110
 ```
+
+### Additional validation
+
+When the [`AdditionalValidation` key](dataset-description.md#additional-validation)
+contains `"Phenotype"` in the `dataset_description.json`,
+the following expectations apply to sessions files.
+
+1.  If there is more than one session for any one participant, then it is
+    REQUIRED to provide a sessions file at the dataset root.
+    The sessions file MUST list all sessions for all subjects
+    across imaging and tabular phenotypic data. If a sessions file is provided, then
+    it MUST begin with a `participant_id` column followed immediately by
+    a `session_id` column. The data dictionary JSON file's `session_id` field
+    MUST include `Levels` with the description of each `session_id`.
+
+1.  When a root-level sessions file is in use, you MUST NOT provide
+    additional sessions files at the participant-level
+    which would otherwise obey the inheritance principle.
+
+1.  Whenever possible, it is RECOMMENDED to also collect acquisition time
+    for tabular phenotypic data and store the time of acquisition of each row
+    inside a column named `acq_time` in the sessions file.
+    This is consistent with how acquisition time is recorded for MRI data
+    and other time-sensitive measurements (for example systolic blood pressure).
+
+1.  When it is needed to preserve participant privacy, you SHOULD record
+    relative acquisition times with respect to the earliest session.
+    Relative session acquisition times MAY be listed as durations from
+    the earliest session (baseline) in days, months, or years
+    using the `acq_time` column.
+
+To read more about the guidelines for tabular phenotypic data and examples,
+see the [Tabular phenotypic data guidelines appendix](../appendices/phenotype.md).
diff --git a/src/modality-agnostic-files/dataset-description.md b/src/modality-agnostic-files/dataset-description.md
index 79d75264c3..c9e73b11cd 100644
--- a/src/modality-agnostic-files/dataset-description.md
+++ b/src/modality-agnostic-files/dataset-description.md
@@ -42,6 +42,7 @@ and a guide for using macros can be found at
       "DatasetDOI": "OPTIONAL",
       "GeneratedBy": "RECOMMENDED",
       "SourceDatasets": "RECOMMENDED",
+      "AdditionalValidation": "OPTIONAL",
    }
 ) }}
 
@@ -164,6 +165,19 @@ Example:
 }
 ```
 
+### Additional validation
+
+The `AdditionalValidation` key MAY be used to opt into additional validation
+to be performed on the dataset beyond standard BIDS validation.
+The value of this field is either a string or an array of strings,
+each of which MUST be the name of a supported additional validation to be performed.
+
+The currently supported values are:
+
+| **Value**     | **Description**                                                                                                        |
+| ------------- | ---------------------------------------------------------------------------------------------------------------------- |
+| `"Phenotype"` | Stricter validation for tabular phenotypic data, as described in the [phenotype appendix](../appendices/phenotype.md). |
+
 ## `README`
 
 <!-- This block generates a file tree.
diff --git a/src/modality-agnostic-files/phenotypic-and-assessment-data.md b/src/modality-agnostic-files/phenotypic-and-assessment-data.md
index d876a38ee7..fb93c5561a 100644
--- a/src/modality-agnostic-files/phenotypic-and-assessment-data.md
+++ b/src/modality-agnostic-files/phenotypic-and-assessment-data.md
@@ -24,9 +24,21 @@ The files can include an arbitrary set of columns, but one of them MUST be
 `participant_id` and the entries of that column MUST correspond to the subjects
 in the BIDS dataset and `participants.tsv` file.
 
-As with all other tabular data, the additional phenotypic information files
-MAY be accompanied by a JSON file describing the columns in detail
+<!-- This block generates a columns table.
+The definitions of these fields can be found in
+  src/schema/rules/tabular_data/*.yaml
+and a guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_columns_table("modality_agnostic.Phenotypes") }}
+
+As with all other tabular data, the additional tabular phenotypic data
+MAY be accompanied by a JSON data dictionary file describing the columns in detail
 (see [Tabular files](../common-principles.md#tabular-files)).
+When the [`AdditionalValidation` key](dataset-description.md#additional-validation)
+contains `"Phenotype"` in the `dataset_description.json`,
+then the additional tabular phenotypic data
+MUST be accompanied by a JSON data dictionary file.
 
 In addition to the column descriptions, the JSON file MAY contain the following fields:
 
@@ -80,3 +92,57 @@ descriptions with a `Derivative` field that, when set to true, indicates that
 values in the corresponding column is a transformation of values from other
 columns (for example a summary score based on a subset of items in a
 questionnaire).
+
+## Additional validation
+
+When the [`AdditionalValidation` key](dataset-description.md#additional-validation)
+contains `"Phenotype"` in the `dataset_description.json`,
+the following expectations apply to phenotypic and assessment data.
+
+1.  It is REQUIRED to aggregate all participant data into
+    one TSV per tabular phenotypic file.
+
+1.  Each tabular phenotypic data TSV file MUST be accompanied by
+    a corresponding data dictionary JSON file.
+
+1.  Whenever possible, it is RECOMMENDED to add `MeasurementToolMetadata` to
+    each `phenotype/<measurement_tool_name>.json` data dictionary.
+    This improves reusability and provides clarity about the measurement tool.
+
+1.  Each measurement tool SHOULD have an independent
+    aggregated data TSV file in which the user collects all subjects, sessions,
+    and/or runs of data as one entry per row (with a row defined by
+    the smallest unit of acquisition). In other words:
+
+    1.  Each row MUST start with `participant_id`.
+
+    1.  Each TSV file MUST contain a `session_id` column when
+        multiple [sessions](../glossary.md#session-entities) are present
+        in the data set regardless of whether those sessions are in
+        the `phenotype/` data, `sub-<label>/` data, or a combination of the two.
+        See the first two examples in [the appendix](../appendices/phenotype.md).
+
+    1.  If more than one of the same measurement tool is acquired within
+        the same `session_id`, a `run_id` column MUST be added.
+
+    1.  To encode the acquisition time for a tabular phenotypic file’s `session_id`,
+        add the `session_id` to the sessions file and
+        include the OPTIONAL `acq_time` column.
+
+    To see this guideline summarized as a table,
+    see [the appendix](../appendices/phenotype.md#to-summarize-this-guideline-as-a-table).
+
+    Furthermore, if you have to add a `session_id` column to the tabular phenotypic data,
+    you then MUST also introduce a session directory to the imaging data,
+    even if only one imaging session has been created.
+    This rule can be considered as "**if anyone uses sessions, everyone uses sessions.**"
+    And vice versa, if imaging data has session directories,
+    all imaging data and tabular phenotypic data MUST have sessions.
+
+    This produces a file in which same-participant entries can take up as many rows as needed
+    according to the smallest unit of acquisition.
+    The combination of values in the `participant_id`, `session_id`, and `run_id` (if present)
+    columns MUST be unique for the entire tabular file.
+
+To read more about the guidelines for tabular phenotypic data and examples,
+see the [Tabular phenotypic data guidelines appendix](../appendices/phenotype.md).
diff --git a/src/schema/objects/columns.yaml b/src/schema/objects/columns.yaml
index f5422abab2..bbfb2882a0 100644
--- a/src/schema/objects/columns.yaml
+++ b/src/schema/objects/columns.yaml
@@ -432,6 +432,13 @@ response_time:
     `n/a` denotes a missed response.
   type: number
   unit: s
+run_id:
+  name: run_id
+  display_name: Run ID
+  description: |
+    A run identifier that corresponds to an existing `run-<index>` entity used in a filename(s).
+  type: string
+  pattern: ^run-[0-9]+$
 sample_id:
   name: sample_id
   display_name: Sample ID
diff --git a/src/schema/objects/files.yaml b/src/schema/objects/files.yaml
index 9c6bc41305..9fa03d972d 100644
--- a/src/schema/objects/files.yaml
+++ b/src/schema/objects/files.yaml
@@ -69,15 +69,19 @@ participants:
   file_type: regular
   description: |
     The purpose of this RECOMMENDED file is to describe properties of participants
-    such as age, sex, handedness, species and strain.
+    such as age, sex, handedness, species, and strain.
     If this file exists, it MUST contain the column `participant_id`,
     which MUST consist of `sub-<label>` values identifying one row for each participant,
     followed by a list of optional columns describing participants.
-    Each participant MUST be described by one and only one row.
+    For participants with multiple sessions, the `session_id` column is RECOMMENDED.
 
     The `participant_id` entries MUST be a superset of all subject directories
     and all `participant_id` entries found among phenotypic and assessment data
     in the `phenotype/` directory.
+    When in use, the `session_id` entries MUST be a superset of all session directories,
+    all tabular phenotypic `session_id` entries found among phenotypic and assessment data
+    in the `phenotype/` directory, and all `session_id` entries found in the
+    [sessions file(s)](SPEC_ROOT/modality-agnostic-files/data-summary-files.md#sessions-file).
 
     Commonly used *optional* columns in `participants.tsv` files are `age`, `sex`,
     `handedness`, `strain`, and `strain_rrid`.
diff --git a/src/schema/objects/metadata.yaml b/src/schema/objects/metadata.yaml
index e6567a57f5..bdbc05c31c 100644
--- a/src/schema/objects/metadata.yaml
+++ b/src/schema/objects/metadata.yaml
@@ -48,6 +48,18 @@ AcquisitionVoxelSize:
     type: number
     exclusiveMinimum: 0
     unit: mm
+AdditionalValidation:
+  name: AdditionalValidation
+  display_name: Additional Validation
+  description: |
+    A string or list of strings of additional validations to be performed on the data,
+    chosen from among a pre-defined set. The currently allowed values are
+    only `"Phenotype"`.
+  anyOf:
+    - type: string
+    - type: array
+      items:
+        type: string
 Anaesthesia:
   name: Anaesthesia
   display_name: Anaesthesia
@@ -2253,6 +2265,7 @@ MeasurementToolMetadata:
     Contains two fields: `"Description"` and `"TermURL"`.
     `"Description"` is a free text description of the measurement tool.
     `"TermURL"` is a URL to an entity in an ontology corresponding to this tool.
+    RECOMMENDED by `AdditionalValidation` of `"Phenotype"` in `dataset_description.json`.
   type: object
   properties:
     TermURL:
diff --git a/src/schema/rules/checks/dataset.yaml b/src/schema/rules/checks/dataset.yaml
index 5fbf91c4a6..d2aca48eff 100644
--- a/src/schema/rules/checks/dataset.yaml
+++ b/src/schema/rules/checks/dataset.yaml
@@ -38,10 +38,8 @@ ParticipantIDMismatch:
     - path == '/participants.tsv'
   checks:
     - |
-      allequal(
-        sorted(intersects(columns.participant_id, dataset.subjects.sub_dirs)),
-        sorted(dataset.subjects.sub_dirs)
-      )
+      length(intersects(unique(columns.participant_id), dataset.subjects.sub_dirs)) ==
+      length(dataset.subjects.sub_dirs)
 
 # 214
 SamplesTSVMissing:
diff --git a/src/schema/rules/dataset_metadata.yaml b/src/schema/rules/dataset_metadata.yaml
index b9c8e74761..0d9cc5cf1e 100644
--- a/src/schema/rules/dataset_metadata.yaml
+++ b/src/schema/rules/dataset_metadata.yaml
@@ -19,6 +19,7 @@ dataset_description:
     DatasetDOI: optional
     GeneratedBy: recommended
     SourceDatasets: recommended
+    AdditionalValidation: optional
 
 dataset_authors:
   selectors:
diff --git a/src/schema/rules/sidecars/phenotype.yaml b/src/schema/rules/sidecars/phenotype.yaml
new file mode 100644
index 0000000000..8b97b40c23
--- /dev/null
+++ b/src/schema/rules/sidecars/phenotype.yaml
@@ -0,0 +1,18 @@
+---
+MeasurementToolMetadata:
+  selectors:
+    - datatype == 'phenotype'
+    - extension == '.tsv'
+    - '!intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])'
+  fields:
+    MeasurementToolMetadata:
+      level: optional
+      level_addendum: recommended by phenotype guidelines
+
+MeasurementToolMetadataRec:
+  selectors:
+    - datatype == 'phenotype'
+    - extension == '.tsv'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+  fields:
+    MeasurementToolMetadata: recommended
diff --git a/src/schema/rules/tabular_data/modality_agnostic.yaml b/src/schema/rules/tabular_data/modality_agnostic.yaml
index 83fd7e14f4..fd6b919b1c 100644
--- a/src/schema/rules/tabular_data/modality_agnostic.yaml
+++ b/src/schema/rules/tabular_data/modality_agnostic.yaml
@@ -2,13 +2,13 @@
 Participants:
   selectors:
     - path == "/participants.tsv"
+    - '!intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])'
   initial_columns:
     - participant_id
+    - session_id
   columns:
-    participant_id:
-      level: required
-      description_addendum: |
-        There MUST be exactly one row for each participant.
+    participant_id: required
+    session_id: recommended
     species: recommended
     age: recommended
     sex: recommended
@@ -16,9 +16,62 @@ Participants:
     strain: recommended
     strain_rrid: recommended
     HED: optional
-  index_columns: [participant_id]
+  index_columns: [participant_id, session_id]
+  additional_columns: allowed
+
+Participants__Additional:
+  $ref: rules.tabular_data.modality_agnostic.Participants
+  selectors:
+    - path == "/participants.tsv"
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+  columns:
+    participant_id: required
+    session_id: recommended
+    HED: optional
+  additional_columns: allowed_if_defined
+
+Phenotypes:
+  selectors:
+    - datatype == 'phenotype'
+    - extension == ".tsv"
+    - '!intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])'
+  initial_columns:
+    - participant_id
+    - session_id
+    - run_id
+  columns:
+    participant_id:
+      level: required
+      description_addendum: |
+        Note that data for one participant MAY be represented across multiple rows
+        in case of multiple sessions or runs, and
+        therefore the entry in the `participant_id` column will be repeated.
+    session_id:
+      level: optional
+      level_addendum: required if sessions are defined in the dataset
+      description_addendum: |
+        A `session_id` column MUST be added to all tabular files in the phenotype directory
+        as soon as multiple sessions are present in the data set
+        regardless of whether those sessions are in the
+        `phenotype/` data, `sub-<label>/` data, or a combination of the two.
+    run_id:
+      level: optional
+      level_addendum: required if there are multiple runs within any session
+      description_addendum: |
+        A chronological `run` number is used when
+        a measurement tool or assessment described by a tabular file
+        was repeated within a session.
+    HED: optional
+  index_columns: [participant_id, session_id, run_id]
   additional_columns: allowed
 
+Phenotypes__Additional:
+  $ref: rules.tabular_data.modality_agnostic.Phenotypes
+  selectors:
+    - datatype == 'phenotype'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+  additional_columns: allowed_if_defined
+
 Samples:
   selectors:
     - path == "/samples.tsv"
@@ -51,26 +104,28 @@ Sessions:
   selectors:
     - suffix == "sessions"
     - extension == ".tsv"
+    - '!intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])'
   initial_columns:
+    - participant_id
     - session_id
+    - run_id
   columns:
-    session_id:
-      level: required
-      description_addendum: |
-        There MUST be exactly one row for each session.
-    acq_time__sessions: optional
+    participant_id: optional
+    session_id: required
+    run_id: optional
+    acq_time__sessions: recommended
     pathology: recommended
     HED: optional
-  index_columns: [session_id]
+  index_columns: [participant_id, session_id, run_id]
   additional_columns: allowed
 
-Phenotype:
+Sessions__Additional:
+  $ref: rules.tabular_data.modality_agnostic.Sessions
   selectors:
-    - datatype == 'phenotype'
-  initial_columns:
-    - participant_id
+    - suffix == "sessions"
+    - extension == ".tsv"
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
   columns:
-    participant_id: required
-    HED: optional
-  index_columns: [participant_id]
-  additional_columns: allowed
+    $ref: rules.tabular_data.modality_agnostic.Sessions.columns
+    acq_time__sessions: required
+  additional_columns: allowed_if_defined