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:
id := "Content-ID" ":" msg-id
Like the Message-ID values, Content-ID values must be generated to be
world-unique.
The Content-ID value may be used for uniquely identifying MIME
entities in several contexts, particularly for cacheing data
referenced by the message/external-body mechanism. Although the
Content-ID header is generally optional, its use is mandatory in
implementations which generate data of the optional MIME Content-type
"message/external-body". That is, each message/external-body entity
must have a Content-ID field to permit cacheing of such data.
It is also worth noting that the Content-ID value has special
semantics in the case of the multipart/alternative content-type.
This is explained in the section of this document dealing with
multipart/alternative.
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.
description := "Content-Description" ":" *text
The description is presumed to be given in the US-ASCII character
set, although the mechanism specified in [RFC-1522] may be used for
non-US-ASCII Content-Description values.
