@david @HistoPol @paul @fediversereport @fediversenews @Bluedepth @darren @juneussell @volkris @davetroy Sorry, I didn't see this at first.

Relays are mainly to merge participating instances' Local Timelines into each one's Federated Timeline, in addition to the posts coming in through normal remote follows, which also helps to fill trending tags. It's a good way to make those discovery features more useful in a group of smaller instances with similar topics, but I'm pretty sure they always increase resource use even if they could be used to send messages to multiple direct recipients at once. (I haven't checked whether the software actually does that, but my hunch is that in many cases it does not.)

What I'm talking about is a server acting as (lower-case) relay for its local users. Let's say you have a thousand followers on mastodon.social. When you make an Unlisted post, then to reach all those followers, your server needs to push that action to that instance only once, and it'll take care to show the post to all interested users by itself.

If you attach an image, your instance needs to give it to the other one only once too, through a similar mechanism, not once for each follower. The other instance should normally cache a there-local copy for a while.

If you @-mention a few people from the same remote instance directly, they can all appear as recipients (to be notified) in the same single server-to-server message.