Hi,

I have not fully read these documents but the first appears to define MIME Extensions, and the second appears to define Content ID's and Message Id's. More specifically the second says

A "cid" URL is converted to the corresponding Content-ID message
header [MIME] by removing the "cid:" prefix, converting the % encoded
character to their equivalent US-ASCII characters, and enclosing the
remaining parts with an angle bracket pair, "<" and ">". For
example, "cid:foo4%[email protected]" corresponds to

Content-ID: <foo4%[email protected]>

Reversing the process and converting URL special characters to their
% encodings produces the original cid.
http://www.ietf.org/rfc/rfc2045.txt

http://www.ietf.org/rfc/rfc2392.txt

Hope this helps