Ok if I jump in? I like advocating for the use of hashtags!
My understanding of hashtags and federation is this:
Via a hashtag you will see all public/global (not: unlisted. (as mentioned already, thanks @FirstProgenitor ! )) toots that your instance "knows of". That is
1. all toots that originated on your instance
PLUS
2. toots from accounts on other instances as far as at least 1 account on your instance follows that account.
(So usually bigger instances (or a small instance with it's users following lots of accounts on other instances) see/know bigger parts of the fediverse.
cc
@extinct
