Two schemes are defined. The first, "mid:", refers to the RFC822 Message-Id of a mail message. This Identifier is already used in RFC822 in for example the References and In-Reply-to field . The rest of the URL after the "mid:" is the RFC822 msg-id with the constant <> wrapper removed, leaving an identifier whose format in fact happens to be the same as addr-spec format for mailboxes (though the semantics are different).
The use of a "mid" URL implies access to a body of mail already received. If a message has been distributed using NNTP or other usenet protocols over the news system, then the "news:" form should be used.