Suppose there are three servers (A, B, C), and A is federated with both, but B isn't federated with C, and there's conversation between people on A and B.
Do people on C only see the A side of the conversation?
I am not fully clear on how threads are handled, but, yes, in principle visibility of users and posts is based on who they choose to federate with. So C would not see any content that originated out of B. Which AFAIK includes replies, etc.
I'm a Mastodon newb and not an expert, I'm sure someone else more informed could give a better explanation of the subtleties.
Server A also sends messages coming from servers it federates with, so those on server C can see server B's messages, but C'ers just can't reply to them and expect B'ers to actually receive those messages.
Suppose there are three servers (A, B, C), and A is federated with both, but B isn't federated with C, and there's conversation between people on A and B.
Do people on C only see the A side of the conversation?