One of the biggest changes will be that both card numbers (credit and debit) as well as the card’s security code and expiry date will all be printed on the back of your combination card. The purpose of this is for you to have one place where you can find the details of both cards easily. When you use your credit card number, you are paying with your card’s credit facility. When you use your debit card number, your payment is charged to the bank account linked to your card.