Why does China Agricultural Bank have a card with 19 digits?

The bank's UnionPay debit cards are all 19 digits. The first card number is 6, which is a card supporting UnionPay. The first digit of all bank cards is the card supporting UnionPay, and the second to fifth digits represent the bank to which they belong. For example, 2220 represents the sixth digit of ICBC and the type of card. For example, "2" is the seventh to tenth digits of a debit card, the eleventh to eighteenth digits are the secondary branch code of the bank to which it belongs, the serial number of the card, and the nineteenth digit (the last digit) is the check number.