Credit and debit card issuers use customer ID numbers to verify that a customer placing an order has a copy of the card with him. These numbers are generally three or four digits long.
Credit and debit card providers use customer ID numbers to make sure the card owner has the card when placing an order; many online locations require users to enter the customer ID number when placing an order. Cards don't store the customer ID number on the magnetic strip, so machines designed to steal and log credit card numbers can't access them. This number is independent from the card number and its expiration date, and it helps prevent fraudulent purchases if somebody steals the card's information.