An issue number is a supplementary number to the account number of certain debit cards, primarily United Kingdom ones such as Switch and Maestro. Because the bulk of the account number is determined by the bank account number, the card number cannot be changed if the card is lost or stolen (without changing the underlying bank account number), and instead the issue number is changed.
Если кратко - дополнительные 2 цифры на картах выпущенных в англии.
На форумах советуют писать 00 если есть поле для его ввода.