Mandatory
If the message is sent by the issuing bank, this field specifies the documentary credit number assigned by the issuing bank.
If the message is sent by a bank other than the issuing bank, this field specifies either the documentary credit number assigned by the issuing bank or another reference assigned by the Sender.
This field must not start or end with a slash '/' and must not contain two consecutive slashes '//' (Error code(s): T26).