Dan,
1. We make the following correspondence between the terms of the SGML
standard and the MIME RFC:
...
SGML SYSTEM identifier => MIME Content-ID
I proposed in an earlier note that the SGML SYSTEM identifier
should be a URI and that a special form of URI (cid:) should
be used to specify a content identifier within a MIME
multipart message.
SGM SYSTEM identifier => URI
This allows SGML to reference anything in the universal
syntax, inclusing now URLs and later URNs, and
other parts of a MIME object using the cid: form.
I think this is important, as external references
are important too, and one will need to mix them.
Tim BL