Service code (card)
The three digits after the expiry date on a magnetic stripe that tell a terminal where the card may be used, whether it has a chip and whether a PIN is required.
Each digit has its own job. The first covers scope and technology: 1 means an international card, 2 an international card with a chip, and 5 and 6 are the domestic equivalents. The second governs authorisation, for instance that the issuer must always be asked online. The third restricts services, such as ATM-only use or a mandatory PIN. The usual suspects are 101 for stripe-only cards and 201 for chip cards.
That leading 2 is what makes a terminal refuse the swipe and ask for the chip instead. A forger who changes 201 to 101 to dodge the chip hits a snag: the service code is part of the input to the card's CVV, so the issuer spots the tampering.