[SIP Beyond VoIP] question about subscribe uris

[Juha Heinanen]

when i start sip-session ua with account jh at test.tutpro.com, it
first registers and then subscribes to its own presence.winfo and
dialog.winfo and then sends presence subscribe to

SUBSCRIBE sip:jh+presence at test.tutpro.com

since that user does not exist, my presence server returns 404 no found.
where does it get userpart jh+presence?  is this + syntax specified in
some rfc?

-- juha