Debit cards have two modes: signature-based debit card, and PIN-based debit cards. When you pay with a debit card in a physical store you swipe your card, and are then instructed to choose between Credit or Debit. If you choose Credit, the payment goes through the card network (e.g. Visa, Mastercard, American Express) to your bank account, your checking account (the account linked to the debit card) gets deducted immediately, and the merchant prints out a receipt which you sign for authorization. The process is very much like a credit card, except that the money comes out of your linked checking account instead of a credit line that the issuing bank (the bank that issued you the credit card) has for you.

On the other hand, if you choose Debit, the payment goes through the Interlink network (which, confusingly, is owned by Visa), and you’re asked to type in your 4-digit PIN number to verify your purchase, and no signature is necessary.

In most cases, a debit card issued by your bank allows you to use both modes: PIN and Signature. It is at the point of sale (the cash register, for example) that you’re prompted to select which mode to use: the Signature mode (by swiping a debit card and selecting, counter-intuitively, “Credit”), or the PIN mode (by swiping a debit card and selecting “Debit”). Merchants prefer you use the PIN number because it costs them less in fees, which is why they make selecting the Signature mode more confusing by marking it as “credit” despite that fact that it’s still a debit card. For the same exact reason (the higher fees), the card’s issuing bank (the customer’s bank) prefers you use the signature method and will often offer you rewards for using it in that fashion.

Online, there’s no way to enter a PIN number, so with cards that are issued by Visa/Mastercard/etc., you can either use a standard credit card, or use a debit card like a credit card, which means that it is used in signature mode (and the merchant gets your “digital signature” on the website by getting all your personal details).