Optional
This field specifies the related reference.
This field must not start or end with a slash '/' and not contain two consecutive slashes '//' (Error code(s): T26).
This field must contain the contents of field 20 Transaction Reference Number of a previous MT 516 or 526 General Securities Lending/Borrowing Message.
The content of field 21 will normally be a reference meaningful to the Sender rather than the Receiver. This is because the Sender will normally include the content of field 20 from the MT 516 previously transmitted rather than received.