This stems from #571 and #584. Essentially, there is some confusion about terminology for different elements of the specification that could be reduced by defining "modality" and differentiating it from "data type". Of course, the actual definition of modality is up for debate.

I think it would be best to define modality as a subtype of suffix. The reasons for this are:

• The term modality_label is used in the anatomical MRI section as a stand-in for all of the anatomical MRI-specific suffixes.
• The "associated modality" entity (mod) refers to the suffix of the file from which a defacing mask was created.
• Suffixes are not, however, all "modalities". See What to call modality directory vs the modality entity? #584 (comment) and Replace terms "modality_label" and "contrast_label" with "suffix" #571 (comment).