Lets assume you have an account on Lemmy.world. Let’s also assume you see some post on Lemmy.ml. And finally lets assume you have a friend that’s actually on Mastodon. When you reply to that post on Lemmy.ml, Lemmy.world sends your reply to Lemmy.ml and then Lemmy.ml tells Mastodon (and all other federated instances) about your reply. But if Lemmy.ml decides to go offline, Lemmy.world has no where to send that reply to, so it’s only kept locally on Lemmy.world. The user on Mastodon can’t see it as their instance wasn’t told about it from Lemmy.ml as it went offline.
Replying to my own post with an example…
Lets assume you have an account on Lemmy.world. Let’s also assume you see some post on Lemmy.ml. And finally lets assume you have a friend that’s actually on Mastodon. When you reply to that post on Lemmy.ml, Lemmy.world sends your reply to Lemmy.ml and then Lemmy.ml tells Mastodon (and all other federated instances) about your reply. But if Lemmy.ml decides to go offline, Lemmy.world has no where to send that reply to, so it’s only kept locally on Lemmy.world. The user on Mastodon can’t see it as their instance wasn’t told about it from Lemmy.ml as it went offline.