Printed on the back of your credit card the 'signature' or 'sig' digits (or 'CVV2' in the credit card business) are the numbers printed on the back of your credit card on the paper of the signature strip, towards the top and right of the strip. There are between three and ten digits. We only require the last three.
Mastercard & Visa have introduced this system to provide greater levels of security to protect the customer. The signature digits are separate from the embossed numbers and from those contained in the magnetic strip. This means that you need to have the physical card to know the number.
We actually have no choice in collecting these digits, Mastercard and Visa will not authorise 'cardholder not present' ie mail order charges without them.