The 3 digit on the back of the card (4 digit on the front of American Express) is called the card security code and is typically referred to as a CVV2 code.  This CVV2 code is used as an extra security precaution for online or other 'card not present' transactions, however per PCI compliance regulations it's a transparent pass on use and cannot be stored anywhere.  What this means is in MyPMS CVV2 should only be used in conjuncture with our MyCard credit card processing, and will only be used when gaining an authorization or attempting to post a charge to the card.  You will not see this value stored anywhere on the booking as we are not allowed to retain it, only pass it with an transaction to the processor for validation.  Using CVV2 will usually get you a better rate from your merchant on card not present transactions, but may add complexity as the processor may decline a transaction with an incorrect CVV2 code.