@evan To start with: the only definition of the inbox I can find is in the context of AP Actors? If that's all that's required to define it, that is definitely not clear. https://www.w3.org/TR/activitypub/#inbox links to the actors section, where 4.0 says that a "Profile can be an actor" but then it goes on to define (in 4.1) actors as having far more than an inbox
But I can't really see the use without also defining a _behavior_ to go along with it, which making that clear in type would be far preferable, IMO?