and nowhere described otherwise:

$> git grep _part                              
src/99-appendices/06-meg-file-formats.md:sub-control01_ses-001_task-rest_run-01_part-01_meg.fif
src/99-appendices/06-meg-file-formats.md:sub-control01_ses-001_task-rest_run-01_part-02_meg.fif

and it is to annotate two parts of the split longer file. In #371 I mention e.g. that _part was (or still is) suggested in multiple BEPs (BEP001 (multiple contrasts) and BEP004 (SWI)) but likely for a different purpose.

I am not sure if we have ATM in BIDS any agreement on annotation of multiple portions (split arbitrarily in temporal dimension) of a larger thing, besides a _run which is different semantically. That is why I was not sure how to address this issue.

attn: @bids-standard/derivatives-electrophys