As someone who creates ActivityPub services, it's often a puzzle to figure out exactly how someone else's service federates. Like, what does service X do when I send it a Delete/Note activity?
In an attempt to be a good citizen, I've drafted up a FEDERATION.md file for my fediverse event organizing service I'm working on. Here it is:
https://github.com/dariusk/gathio/blob/master/FEDERATION.md
I've also started a meta-discussion about how we might start documenting this stuff generally:
https://socialhub.activitypub.rocks/t/documenting-federation-behavior-in-a-semi-standard-way/453