If the subscriber is aware of a namespace of interest, it can send SUBSCRIBE_ANNOUNCES to publishers/relays it has established a session with. The recipient of this message will send any relevant ANNOUNCE or UNANNOUNCE messages for that namespace, or subset of that namespace.
A publisher MUST send exactly one SUBSCRIBE_ANNOUNCES_OK or SUBSCRIBE_ANNOUNCES_ERROR in response to a SUBSCRIBE_ANNOUNCES. The subscriber SHOULD close the session with a protocol error if it detects receiving more than one.
The receiver of a SUBSCRIBE_ANNOUNCES_OK or SUBSCRIBE_ANNOUNCES_ERROR ought to forward the result to the application, so that it can make decisions about further publishers to contact.
An UNSUBSCRIBE_ANNOUNCES withdraws a previous SUBSCRIBE_ANNOUNCES. It does not prohibit the receiver (publisher) from sending further ANNOUNCE messages.