The credit card issue number is another name for either the three or four digit set of numbers that identifies your credit card type and account.
Three Digits
Visa and Mastercard cards have a three digit issue number located on the back of the card either on or near the signature strip.The issue number is also commonly referred to by Visa as the CVV or Card Verification Value and by Mastercard as the CVC or Card Validation Code. Discover refers to their three digit code as The Cid or Card ID.
Four Digits
In contrast to Visa, Mastercard of Discover, American Express cards have a four digit issue number. This four digit number can be found on the front of the card, on the top right of the card number.
Purpose
The purpose of the issue number is for identification and for protection when you use your credit card for transactions, especially when online. When you are prompted to provide the issue number or CVV, CVC or CID, doing so proves that you are actually present at the time of the transaction. The code is never stored, however, is transmitted to your credit card issuer's gateway, where as a result of its input, a validation code is received by the vendor.