Most replies here are correct. To clarify and summarize:
- The source of truth for a magazine/community is the server name that appears after the magazine name.
e.g. kbinMeta@kbin.social <--- the source of truth is kbin.social.
2. ActivityPub and the Fediverse is a "Push" model. What does this mean?
Imagine subscribing to a real-world newspaper or magazine with home delivery(few these days will actually remember this, but try to imagine at least). You will get all new issues delivered to you from the moment you became a subscriber, but you don't get copies of all the newspapers or magazines they have ever printed delivered to you. You only get things moving forward. That's the same with the Fediverse. After you subscribe or follow something, you will get all the new content moving forward, but not what has been created so far.
- To extend #2, it's a "push once" model. What does that mean? It means that if I create content from my instance (which is not kbin.social) to the magazine kbinMeta@kbin.social, my content will get pushed to kbin.social. Kbin.social, however, will not "re-push" that content to everyone that kbin.social knows is subscribed to the magazine.
So how does my new content that I created in kbinMeta@kbin.social show up on other instances that are not kbin.social? I thought you said your content only gets pushed once?
Correct. However, it's not quite as simple as my instance pushing just to kbin.social. Strictly speaking, (and this is based on experience with other platforms, not specifically how kbin works since I haven't verified this for kbin 100%) when I create the content, my instance will push to kbin.social and all other instances (not users) that my instance knows are also subscribed to specifically kbinMeta@kbin.social. So my instance actually knows a subset of the instances that are subscribed to kbinMeta@kbin.social and will push the new content to each of those other instances. My instance, however, won't necessarily know all the other instances that are subscribed to kbinMeta@kbin.social. As a result, some instances won't see my new content because it wasn't pushed to them.
- As a result, to let users know about this potential gap, not only does it mean that older content doesn't automatically show up, it also means that not necessarily all new content will show up either.
Note on #3: I haven't fully verified this. This statement is based on how other, non-kbin instances handle federation. This is how "likes" work across platforms like Mastodon, Calckey, etc. I see no evidence (yet) that this is any different for kbin.