What is your Card Identifier # or CVV2/CID?
  This number is recorded as an additional security precaution to ensure that your credit card information is not being used fraudulently.

This number can be located in different spots on your cards. On Visa, MasterCard and Discover the Card Identifier # or CVV2/CID number is the last 3 numbers on the back. On American Express it is  located on the front.
  Visa, MasterCard, and Discover
CVV2 stands for Card Verification Value, sometimes referred to as CVC2 (Card Verification Code). It is the last 3 digits following the credit card account number within the signature panel on the back of the credit card.
 
American Express
CID stands for Cardholder Identification Code. It is the 4 digit value above and to the right of the credit card account number on the front of the credit card.
   Why is this needed?
  This is an additional measure to protect our customers from fraudulent transactions. The CVV2/CID can only be found on the card itself. A transaction created with a valid CVV2/CID means that the person requesting the transaction must have the actual card in his/her possession and is the valid card holder.
  This verifies to the merchant that you are the card holder.  With out this information the merchant assumes the risk for the sale. Many financial institutions now require merchants to provide this code for verification. Without proper code, merchant may decline to process your on line order.