What is a Unique Card Code (CID)?
American Express® credit cards have a 4-digit code printed on the front side of the card above the number, referred to as the CID (or Unique Card Code). It is printed flat, not embossed like the card number.
NOTE: This is the second code of the Card Security Code
Electronic Merchant Systems Payment Processing Glossary :
Defining key payment processing and merchant services industry terms to help business owners make better decisions.