The communities are just relays. There's no community event to limit the spread of a note. If you don't want more than one relay to hold the note, write it to a private relay.
Discussion
You can't see this event, or its replies, for instance. But someone from the #GitCitadel team, like nostr:npub1wqfzz2p880wq0tumuae9lfwyhs8uz35xd0kr34zrvrwyh3kvrzuskcqsyn can add that relay to his Jumble, select it from the list, and read and write to the discussion.
#Alexandria will also be getting Discussions, but we're doing a big revamp, at the moment, to get v0.1 out, so I implemented it here, first.