This document defines the "cid:" and "mid:" URL schemes for referencing messages and their parts. The "cid:" scheme is used within multipart messages, like emails, to reference embedded content such as images. The "mid:" scheme allows referencing entire messages or specific parts within them using their Message-ID and Content-ID. It details the syntax for these URLs, emphasizing the importance of unique Content-IDs and the handling of encoded characters. The specification requires implementations to support the full "mid" URL form (message-id/content-id) for interoperability.
