The way I understand the general URL format specification in
draft-fielding-url-syntax-07.txt, any URL can have a fragment added to it.
Thus, the specification of a specific URL type, like the specification
of the cid and mid URL type in draft-ietf-mhtml-cid-v2-00.txt does not
need to say anything about fragments.
Yes, the presence or absence of a fragment is a property of the reference
and not the URL scheme. One thing that is useful for a URL scheme spec
to say is whether or not it can be used as a hypertext (retrieval style)
reference, since that determines whether or not a fragment would be
meaningful.
....Roy