So then could you have, say, a 3D printing "subreddit" on lemmy.ml as well as lemmy.world? And if you had no idea of the one on lemmy.ml, you would never see the posts submitted there, correct?

Edit: I think I understand now. Someone correct me if I'm wrong. You would be able to see it on your instances "all" page only if one other person from your instance went and subscribed to that other community (from the other instance). So it essentially grabs the content from that other instance and shows it on yours, without actually having to go and sign up for that other instance.