feat(3): add terminology for root resources and collections

aep/general/0003/aep.md.j2

@@ -80,6 +80,12 @@ addresses.
 Either a programmatic client or a user that consumes an API. This term should
 be used when a statement refers broadly to both programs and users.
 
+### Collection
+
+A collection represents a discrete set of resources of a specific type. A
+resource may have one or more collections, with each collection having its own
+parent.
+
 ### Declarative Clients
 
 Declarative Clients, also known as Infrastructure as Code (IaC), describes a
@@ -99,10 +105,27 @@ Examples of complexities that declarative clients abstract away include:
 
 [Terraform][] is an example of such a client.
 
+### Root collection
+
+A [collection](#collection) that does not have any parent collections. For
+example, `/users/`.
+
+### Root Resource
+
+A root resource is a resource for which a collection exists that has no
+parents. In other words, it is a resource that has a collection that appears at
+the root of a resource hierarchy.
+
+A root resource may also appear as a child of another resource: for example, a
+music streaming services may have global shared playlists (e.g. "top
+trending"), but may also have playlists nested under a user. In this case, a
+playlist is still considered a root resource, since the collection with the
+glocal shared playlists has no parents.
+
 ### Schema
 
-A schema describes the structure of the request or response of an [API
-method](#api-method), or a [resource](#api-resource). It refers both to an
+A schema describes the structure of the request or response of an
+[API method](#api-method), or a [resource](#api-resource). It refers both to an
 OpenAPI schema as well as a protobuf message.
 
 ### User

aep/general/0121/aep.md.j2

@@ -28,9 +28,10 @@ When designing an API, consider the following:
 A resource-oriented API **must** be modeled as a resource hierarchy, where each
 node is either a simple resource or a collection of resources.
 
-A _collection_ contains resources of _the same type_. For example, a publisher
-has the collection of books that it publishes. A resource usually has fields,
-and resources may have any number of sub-resources (usually collections).
+A [_collection_](./0003#collection) contains resources of _the same type_. For
+example, a publisher has the collection of books that it publishes. A resource
+usually has fields, and resources may have any number of sub-resources (usually
+collections).
 
 **Note:** While there is some conceptual alignment between storage systems and
 APIs, a service with a resource-oriented API is not necessarily a database, and