External-Description:  A brief explanation of the contents and provenance.
External-Identifier:  A sender-supplied identifier for the bag.
Bag-Group-Identifier:  A sender-supplied identifier for the set, if any, of bags to which it logically belongs.  This identifier must be unique across the sender's content, and if recognizable as belonging to a globally unique scheme, the receiver should make an effort to honor reference to it.
Internal-Sender-Identifier:  An alternate sender-specific identifier for the content and/or bag.
Internal-Sender-Description:  A sender-local prose description of the contents of the bag.