It is currently somewhat unclear when to apply the soundtrack style.

https://musicbrainz.org/doc/Style/Specific_types_of_releases/Soundtrack says "This guideline covers release groups of type "Soundtrack", and the releases, recordings, and works associated with them."

https://musicbrainz.org/doc/Release_Group/Type says "A soundtrack is the musical score to a movie, TV series, stage show, computer game etc."

This doesn't really indicate whether that means only recordings which were actually used in the movie etc. or whether it covers any performance of a "musical score" (which I understand to be any performance of a work which was originally created for a movie etc.)

Some considerations:
1. Soundtrack albums consisting of popular music might not fall under soundtrack, if it only includes works which are part of the "musical score" (most pop music is not created for the movies in which it is used)
2. Third-party performances of "Soundtrack music" ("The music of $COMPOSER" sort of albums.) – these are kind of weird under the current soundtrack style. Either they should be somehow excluded from the soundtrack guidelines (and referred to classical isntead), or Soundtrack Style should be improved it works better for them (specific problem is that if there is a composer+performer on the cover, then the guidance is to apply both to the tracks as well, which is ugly and not really what was intended – though it does have the side-effect of hiding the "ugly" track artist from display in most cases.)