Mandatory
This field contains the reference to the associated (enveloped) EDIFACT message.
This field must not start or end with a slash '/' and must not contain two consecutive slashes '//' (Error code(s): T26).
The content of this field should reflect, up to a maximum of 16, the significant characters of the Document/Message Number (Data Element 1004) in the Beginning of the Message (BGM) segment of the EDIFACT message embedded in field 77F. When more than one MT 105 is sent to convey the EDIFACT message, the content of field 21, must be the same for each MT 105 in the series.
This requirement is to facilitate the unambiguous re-association of the separate 'pages' of the transaction, and also to provide a direct link to the EDIFACT message in case of further processing, eg, cancellation.