There is only a difference if you are signed into multiple accounts at the same time. All Subscribed is a combined feed of all of your account's subscribed feed in one feed.

Actually I think it would be good to hide this feed and only show it if the user is signed in more than 1 account.

Three letters?

Seriously though, its usually local instance vs any instance. Not a summit user, but that is how I interpret it in the context of Lemmy. If I am wrong, at least I got to make a snarky comment. ;)