In constructing a high-level user agent, it may be desirable to allow one body to make reference to another. Accordingly, bodies may be labeled using the "Content-ID" header field, which is syntactically identical to the "Message-ID" header field:
Content-ID := msg-idLike the Message-ID values, Content-ID values must be generated to be as unique as possible.
6.2 Optional Content-Description Header Field
The ability to associate some descriptive information with a given body is often desirable. For example, it may be useful to mark an "image" body as "a picture of the Space Shuttle Endeavor." Such text may be placed in the Content- Description header field.
Content-Description := *textThe description is presumed to be given in the US-ASCII character set, although the mechanism specified in [RFC- 1342] may be used for non-US-ASCII Content-Description values.