In the case of Ditto, whether your posts can go a community (like this Gleasonator demo community) is determined by your NIP-05. That's your authority to post.

If there was such a thing as a NIP 5.1 (a NIP 5.0 for communities) and you could have multiple of those, it seems at first glance that the end result of such a thing would be pretty similar to the end result for communi-keys. As in you could publish to multiple communities at once, one community could span relays, there'd be moderation, etc.