Some functionality in Mastodon required some additions to the protocols to enable seamless federation of those features:
The Salmon slaps have the exact same structure as standard follow/unfollow slaps, the verbs are namespaced:
http://mastodon.social/schema/1.0/block
http://mastodon.social/schema/1.0/unblock
Statuses can be marked as containing sensitive (or not safe for work) media. This is symbolized by a <category term="nsfw" />
on the Atom entry
Statuses that are intended to be listed publicly on e.g. "whole known network" or "public" timelines contain a <link rel="mentioned" href="http://activityschema.org/collection/public" ostatus:object-type="http://activitystrea.ms/schema/1.0/collection"/>
. Conversely, statuses which do not contain that, are intended to be low key, unlisted