It's definitely a gray area. This is why we waited all year for the ISU to clarify
their definition of a musical. (It only matters what the definition is per the ISU. I mean, I would never consider disco to be swing music; but it qualified as swing during the 2016-17 season because the ISU included disco within
its definition).
I'm pretty sure that if it was an actual, on-stage musical, then it's fine.
In and of itself, how do you clarify the difference between an on-stage play with a song in it and an on-stage musical? I doubt it is enough for it to have been on stage with music. I believe there would still need to be some music that is used outside of a performance-only setting. (Another poster elsewhere defined a musical as being a story in which the characters do not
know they are singing. I think that's not a bad definition either). Many films that are turned into stage productions have extra songs
added to the score or the soundtrack is then performed
by the characters so it is more likely that if there is a stage production version then it would meet the requirements.
Wouldn't Cabaret then not be considered a musical? Unless I'm forgetting something, I believe all of the songs in it are diegetic.
I don't know Cabaret well enough to answer you as I've only seen it once. I thought there was a song in the dressing room in the stage production I saw; but I cannot replay the whole production in my brain.
Same with Chorus Line... maybe? All of the music in that was in context of an audition, wasn't it?
A Chorus Line, I have thought about. At the very least, the song with Cassie backstage would have to qualify as an out of a performance setting, don't you think? (I mean she's singing while other people are out there on the stage performing. She's singing to herself, and she's not rehearsing). Though I would agree that the fact that the songs aren't prepared for the audition probably should qualify it on its own. My understanding is that skaters are welcome to use any songs from a musical as long as the larger production as a whole is defined as a musical via the ISU's definition.
So, what about Victor/Victoria?
I've never seen Victor/Victoria so I haven't the foggiest.
Would All that Jazz be suitable?
From Chicago? Yes. Because Chicago qualifies as a musical via the ISU definition. There are other songs in it that are outside of a performance setting. Therefore, skaters are welcome to select anything from the score (though they will have to meet the beats per minute rules like always).