@sgryphon Every user has a public RSS feed. "Subscribe" parses that feed and imports it into your timeline. "Follow" fetches the posts using the ActivityPub protocol instead.

If you subscribe to someone, it's invisible to everyone except you:
you will NOT see his follower-only posts
it will NOT increase his follower count or your follow count
he will NOT have to approve your request if his account is private.

A little while ago, QOTO was having a technical issue where following people on other instances was silently failing. Subscribing was handy in that circumstance for those who didn''t care about missing followers-only posts.