Mandatory
If the Receiver of this message has previously sent an MT 768 Acknowledgement of a Guarantee Message or its equivalent, this field contains the contents of field 20 Transaction Reference Number of the acknowledgement.
If no acknowledgement has been previously received, this field will contain a reference which is meaningful to the Receiver, eg, the guarantee number.
This field must not start or end with a slash '/' and must not contain two consecutive slashes '//' (Error code(s): T26).
If no meaningful reference is available, the code NONREF must be used.