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.
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.