Is <source_entities> sufficient or should be extended to include _mod- when necessary?

[yarikoptic]

ATM (v1.4.0-373-g2f0f61b) BIDS says

-   Each Derivatives filename MUST be of the form:
    `<source_entities>[_keyword-<value>]_<suffix>.<ext>`
    (where `<value>` could either be an `<index>` or a `<label>` depending on
    the keyword; see [Definitions][definitions])

-   When the derivatives chain involves outputs derived from a single raw input,
    `source_entities` MUST be the entire source filename, with the omission of
    the source suffix and extension.

so the suffix (and extension) are stripped. It is possible (even if not too common) to have files with multiple suffixes (e.g. _T1w and _PD). That creates a collision while establishing any derivative data on such sets of files (ref: #518, #519; also I think mentioned as possible problem within BEP014 to store transformations). I wonder how we could/should resolve it?

There is an odd _mod- entity used ATM only for _mod-<priorsuffix>_defacemask. Shouldn't <source_entities> description in above to at least allow (if not to RECOMMEND to remove possible ambiguity and to make it more explicit) to add _mod- entity with the original suffix?

[effigies]

I have no objection to extending mod- in this way.

[yarikoptic]

Flipping the table: what to do if it was a _mod-T1w_defacemask and I created a derivative file from it, what should <source_entities> become?