It depends on the store. Some provide you with a barcode or a QR code that the cashier at said store can scan to apply your gift card balance. Some provide a long number like a credit card number that the cashier has to type in. Some also provide an additional 4-digit PIN to go with barcode or long number. I've had mixed results trying these cards at self-service checkouts - generally the ones with a barcode or QR code which can be scanned work fine, but the cards which have a long number to type in need some special option that only cashiers have access to.