Not sure what local timeline does exactly. I'm guessing it's not geographically local, but rather users on the same instance?
@Itsamecj Correct. Also, federated timeline is local + other instances' users' messages that have been sent to your instance because at least 1 user is following them, not the Entire Firehose of Toots.
@Itsamecj Indeed. It's one of the weirder parts of gnu social, and it's going to be interesting to see how it plays out this time.