For the federating its a new kind of AP actor. I’ll be putting in a FEP for it in the near future, but its basically a “Group” that only cares about the “Following” collection.

You can see example json for the AP interactions here: https://codeberg.org/rimu/pyfedi/src/branch/main/docs/activitypub_examples/feeds

The AP interactions for a Feed are:

• Send a Follow request for a Feed
• Accept a Follow request (this is automatic for public feeds)
• Reject a Follow request (this is automatic for private feeds)
• Announce an Add of a Community to a Feed
• Announce a Remove of a Community from a Feed
• Send a Delete of a Feed to subscribers