Let’s say Andy is on the Andy-instance, Beth is on the Beth-instance, and Charlie is on the Charlie-instance. The Charlie-instance has a community called Something-Cool, and all three people are subscribed to it. Beth makes a new post in Something-Cool. The “real” version of the post is on Charlie-instance because that’s where the community is, but the Lemmy software on Andy-instance and Beth-instance makes a copy of the post locally. Andy comments on the post, which happens on his local instance, but the software synchs up his comment with the “real” version of the post on Charlie-instance.

Now let’s say that Andy-instance defederates from Charlie-instance. That means that the copying and syncing no longer happen between those two instances. There’s already a copy of Beth’s post on Andy-instance, and that doesn’t get removed, but if anyone else comments on that post, Andy will never see it because his instance isn’t connected to the “real” version. He could make another comment on the post, because it’s still on his instance, but his comment won’t get synced to the “real” version, so it will never get synced to any other instances either, because they always take from the “real” version. Other people on Andy-instance will see Andy’s new comment because it’s on their home instance.