Mob is an unnamed/no face character - to give an example, there is a drawing of a character getting fucked but the person fucking the character doesn't have any face (you only see their body). The unseen person is what people call as "mob".

Orgy means a group of characters having sex with each other (no fixed partners). This is different from gangbang, which is multiple people having sex one person.

Free use is a character basically being "free use (for sex)" for their partner...or anyone I think, no matter the situation the free use character is in...how you approach this prompt is up to you though!