I think this means that a variable recorded in the card (known as the PIN counter) is reduced by 1 if the terminal is capable of writing to the card. This might be a way of recording failed attempts to enter the PIN number. If three tries are allowed, the PIN counter would have a maximum value of 3, which would be reduced by one each time a wrong number was entered. When the value = 0, the card is probably retained by the ATM.