Contains a unique transaction identifier.
This field contains a unique merchant-generated transaction identifier. This identifier must be 20 bytes in length and statistically unique. The component will then Base-64 encode the value for you prior to sending the value in the request.
Note: When submitting the TransactionId (XID) to your payment processor after ThreeDSecure authentication is completed, you may be required to submit the value in the 28-byte Base-64 encoded format. In such a case you can use the value contained within AuthenticationXID.