Card Verification Number (CVN) is an anti-fraud measure used by some banks to prevent payments from generated card numbers. The CVN is printed on the physical card, and is randomly assigned, therefore cannot be auto-generated. 

On VISA, MasterCard, BankCard and Diners Club cards, the CVN is a 3-digit number printed at the end of the credit card number in the signature strip on the back of the card. On American Express cards, the CVN is a 4-digit number printed above the card number on the front of the card.