At the moment the general agreed style is as follows:

A recording can have a work created for it (strictly speaking 90% of all recordings should have works linked, excluding things like non-music tracks)

A soundtrack (for a movie, video game etc.) work can be created, and have its "elements" (i.e. songs or score works that appear in the soundtrack) linked to it.

This works fine for the moment but is slightly messy, see:

https://musicbrainz.org/work/360f72f3-6482-4731-9dad-fc04639f2403

The soundtrack for this movie contains both score items and original songs used in the film; this means that the soundtrack work entity looks slightly messy.

My suggestion is to expand the Part of attribute possibly to allow specification as to whether the item is part of the Score; this should then allow the element work parts to split on the soundtrack entity page into Score items and Soundtrack items.