I said… imagine back to when there were the two main fourms for our favorite author? Now imagine if you could sign up as a user for one… but the forums can talk to one another so you can post on the other too, but your username will reflect the domain of the one you made your username on.
And defederation is when the two factions of fandom get into some fandom drama and decide not to let members of the other board talk to them anymore, lol.
It’s like one part forum, one part irc with distinct chat rooms around a topic, and one part signing up for a new email address, where the place you do it becomes part of your email address.