@Doll_Tow_Jet@kbin.social I don't know if that's the official word for it but I'm using "backfilling" to refer to loading the history of a person or group after first federating with it.
the outbox is a special collection (list in activitypub speak) that's intended to work kinda like an old school email outbox where you put messages to be delivered and it would deliver them to the inboxes of people (and servers that were offline at that time would later pull them in from the outbox)
or, well, that's what the spec says. nobody uses it like that in reality because activity pub was intended for a completely different kind of social media than how it ended up being used (it seems to expect more "everything apps", including an entire client api that would completely abstract away the instance into nothing more than a dumb pipe for activities unlike the current reality where instance software dictate what you can do)
this probably confused you even more but it's getting pretty late here so I can't words good, sorry!