These are encapsulations of several body parts in the one message. MAY contain HTTP metainformation header fields which ARE significant. This is normally used in mail to send different content-type variants when the receiver's capabilities are not known. This allows, for example, pointers to be returned by a name server to a set of instances of an object. For example, the first part could be an HTML document, and the included bodyparts could be the inline images mentioned within the text.
Examples are a picture with audio commentary, etc.
This allows control of caching mechanisms, and also allows for the periodic refreshing of displays of volatile data. This does NOT imply that the original object will cease to exist. A unique identifier for the message. No two documents, even if different versions of the same live document, may have the same Message-id. This gives a URI with which the object may be found.