Other servers can and should use Alice's `replies` collection to see which replies she has consented to. They can and should obscure or hide altogether replies that aren't in that collection.
Mastodon doesn't do either of these things, by the way. It doesn't let you reply to Alice's followers, and it doesn't use the `replies` collection for showing and hiding replies. It's too bad; these are really valuable features of ActivityPub.
@evan I think this is one of the problems with Mastodon being the go to for people in the fediverse, and is also why I've been looking things up and trying to work out how the good ol concept Google+ had could work well in the fediverse, and also supporting all the features of ActivityPub instead of using its own thing, and I even had the idea that if I can get a service like that off the ground, I'd be happy for a non-biased nonprofit organisation to take over. A name for it I thought of is Fedi+ and if eventually that was to be ran by something like the Social Web Foundation or something, that could really help the fediverse at large.