The issue number of a credit card is an additional set of numbers found at the end of the account number, and is used to allow for changes that may be needed if the card is lost or stolen. The issue digits are part of the full account number.
For most cards including Visa, MasterCard, and American Express, the card issue number is located on the signature strip on the back of the card after a four-digit number. The four-digit number represents the last four digits of the 16-digit card number. Cardholders are usually asked to provide the 3-digit issue number for verification purposes during online or phone transactions. American Express issues numbers that are four digits in length.