Say a user from Instance B commented on a post on Instance A, and a user on Instance A saw it. If the user on Instance A were to report it, would that report be seen by the admin(s) of Instance B, or would it only be visible by the admin(s) of Instance A?
If you look at it from a different angle and ask: who might be interested by a user being reported, given that each instance operate independently? The answer is all of them.
The rest comes naturally: obviously if the account is banned at the source it’s effectively banned globally. If it’s banned on the community’s instance, then you won’t see that user there but might on other instances. And your instance can ban the user, in which case they’re freely posting on other instances but you won’t see it from your perspective.
Oh, I totally follow the logic of why each group would want a report, but I would still like to have an official source for the functionality besides anecdotes, if possible.