Priority header values

Sorry to interrupt the MIME discussions, but I cannot, for my life,
find where the Priority: mail header is defined.  I've searched through
the RFC archives, and only find references to it, claiming it is defined
in 822.  But it is not; at least not in my copy.  Could some kind soul
point me to where I can find a defined range of values for this
beasty?

        thanks,
--

        -- Frank Bieser (frankb(_at_)hal(_dot_)com)