Content-Id
The second scheme, "cid:", is similar
to "mid:" , but makes reference to
a body part of a MIME message by
the value of its content-id field.
This allows, for example, a master
document being the first part of
a multipart/related MIME message
to refer to component parts which
are transferred in the same message.
Note
Beware however, that content identifiers
are only required to be unique within
the context of a given MIME message,
and so the cid: URL is only meaningful
with the context the same MIME message.
For a reference outside the message,
it would need to be appended to the
message-id of the whole message.
A syntax for this has not been defined.