Ok, great, I think we’re on the same page. My concern is that I don’t see how phenotype can be linked specifically to sessions from the documentation here: https://bids-specification.readthedocs.io/en/latest/03-modality-agnostic-files.html#phenotypic-and-assessment-data. You could include ‘ses-{on|off|hc}’ in the name but my concern is that this is unstructured/not in the specification so it seems to me likely to cause ambiguity. What I like about the way it is structured now is that the data only relevant to the session of interest is in that particular session directory. In my interpretation, that is a general principle of BIDS: data/metadata ideally goes in the most upstream directory in which it applies to everything downstream. For example, it wouldn’t make sense to me to have a bids_root/task-rest_ses-on-beh.json file because it doesn’t apply to other sessions which are downstream of the root directory, in that case it would be better to have a sidecar for each behavior file in the session folder it applies to even if it’s a bit redundant for clarity. Maybe that’s not essential, but I think it’s better to err on the side of putting metadata in the same directory/paired behavior structure as the data rather than putting everything upstream and relying on someone to sort out which downstream structures it applies to.