Content-ID and Message-ID Uniform Resource Locators
The Uniform Resource Locator (URL) schemes, "cid:" and "mid:" allow references to messages and the body parts of messages. For example, within a single multipart message, one HTML body part might include embedded references to other parts of the same message. [STANDARDS-TRACK]
Normative requirements
This document does not use the RFC 2119 keywords. It states its requirements in ordinary prose, which is why nothing is listed here rather than because nothing was found.