[SIP Beyond VoIP] question about user+event request uri userparts

[Juha Heinanen]

this is resend, because i had used the word "subscribe" in on subject
line and the message was blocked.  i now try with another subject line.

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