A system serial number on a Nintendo Gift Cards is a unique code associated with the card that can be used to identify the card and track its purchase and usage. This serial number is printed on the back of the card, and is typically a 12-digit alphanumeric code. The serial number is used to check the balance of the card, and can also be used to trace purchases and confirm the authenticity of the card. This is to ensure that the gift card is being redeemed on the correct console and to prevent fraud.