In the Work style guideline, there's this example:

John Williams' soundtrack for Star Wars, Episode V: The Empire Strikes Back is set to "Soundtrack", but the Imperial March should not be.

After discussion, it was explained that the reason for this is because the "Soundtrack" work type is only intended the entire soundtrack, not the individual cues. (Side thought, "film cue" would be good candidate for work type.)

There's no explanation given for why that should be, so users don't know how to apply that example to other works. We really just need one more sentence explaining the intent of the example.