When adding cards to a device with Apple Pay, a Device Account Number is created in place of your card number. This number is used when you make a payment with Apple Pay (or a refund of that payment), so that your card number isn't shared with the merchant and won’t appear on the receipt. Each time you add a card to a device with Apple Pay, a new Device Account Number is created.
If you are using Apple Pay to obtain a refund, you may have to provide the last 4 digits of your Device Account Number. You can find your Device Account Number on the card details screen in your Apple Wallet.