Sizing
Card Security Code Explanation
The card security code is a three or four digit security code imprinted on the back of your credit card to help merchants verify that the customer has a legitimate card in hand at the time of the order.
The merchant asks the customer for the security code and then sends it to the card issuer as part of the authorization request. The card issuer checks the security code to determine its validity, then sends a result back to the merchant along with the authorization.