Right now just using like an x-client header i think, not super secure. Not walled. Users can post and read from other relays as well. e.g. when you post there is a checkbox that says “share to all relays”.
Still working out the details of how it interacts with the rest of nostr, but i think the way it may end up is, the feed only pulls top-level notes from the stemstr relay, but all other events, replies, reactions, etc. are pulled from other relays as well.
That way you have a feed of only app-specific content, but it still meshes with the rest of nostr.