DOC: Add notes on processing level to outputs.rst

Author: effigies

docs/outputs.rst

@@ -62,6 +62,28 @@ This layout, now the default, may be explicitly specified with the
 For compatibility with versions of fMRIPrep prior to 21.0, the
 `legacy layout`_ is available via ``--output-layout legacy``.
 
+Processing level
+----------------
+As of version 23.2.0, fMRIPrep supports three levels of derivatives:
+
+* ``--level minimal``: This processing mode aims to produce the smallest
+  working directory and output dataset possible, while enabling all further
+  processing results to be deterministically generated. Most components of
+  the `visual reports`_ can be generated at this level, so the quality of
+  preprocessing can be assessed. Because no resampling is done, confounds
+  and carpetplots will be missing from the reports.
+* ``--level resampling``: This processing mode aims to produce additional
+  derivatives that enable third-party resampling, resampling BOLD series
+  in the working directory as needed, but these are not saved to the output
+  directory.
+  The ``--me-output-echos`` flag will be enabled at this level, in which
+  case the individual echos will be saved to the working directory after
+  slice-timing correction, head-motion correction, and susceptibility
+  distortion correction.
+* ``--level full``: This processing mode aims to produce all derivatives
+  that have previously been a part of the fMRIPrep output dataset.
+  This is the default processing level.
+
 Visual Reports
 --------------
 *fMRIPrep* outputs summary reports, written to ``<output dir>/fmriprep/sub-<subject_label>.html``.