Film lot dressing room.

That only appears if your sim is doing the Prime TV and Movie gigs.
Auditions that require skill level to be on 5 are for Prime TV gigs and auditions that require skill level to be on 7 are for Movie gigs.

"Stars" dressing room?
Is there another dressing room or something?

The only one I remember is the one that unlocks at a certain level in the Acting Career, it had a TV, a Couch, a Bathroom, and a sort of Clothing Hanger/Dresser thing. And it was really just a room right next to the Hair/Makeup prep room.

I don't remember it ever being LOCKED, like door-wise, but before I was a certain level in the Acting Career it just straight up didn't exist. (the room just had huge crates inside it).

The door is for the dressing room which is only active during the gig. If you stay in the studio late, you need to delete it and replace it with a regular door. Then you can come and go as you please. The lot will reset itself after you leave so no worries! Hope this helps! I figured it out after my poor sim needed a shower at the end of a gig.

Thanks for the replies....I sussed it out :)