What does the Card Identification Number (CID) do?

The Card Identification Number (CID) plays a crucial role in enhancing the security of card transactions. Specifically, it acts as a check value that helps verify the authenticity of the card. This unique number is typically found on the back of credit cards and is not stored on the magnetic stripe or chip. When a cardholder makes a transaction, the CID is often required as an additional verification step, particularly for online purchases. By requiring this number, merchants can reduce the likelihood of fraud, as it confirms that the person making the transaction possesses the physical card, making it more difficult for unauthorized individuals to use stolen card information.

The other options do not accurately represent the function of the CID. For instance, the CID is not a regular PIN, which refers to a personal identification number used for ATM or in-person card transactions. Moreover, while it is true that the CID is a unique number, it is not simply a 4-digit number nor is it specifically used for tracking purchases—these functions belong to different mechanisms within the payment processing system. Overall, the CID's primary purpose is to enhance security by verifying that the card is legitimate during a transaction.