It is strongly recommended that all transaction requests contain a unique identifier per request generated by the POS, included in the message request header in the ClientTxnId field and echoed in the response. This value can be used to initiate reversals or search for transactions. The value should be unique for a minimum of one year to search transactions.

ClientTxnId is required to initiate a reversal for any financial transaction in the event that response in not received (POS Timeout).

The ClientTxnId values must be unique for each transaction request for a DeviceId to ensure that any Reversal requests initiated by ClientTxnId are able to identify the correct transaction to be reversed.