The current 16-digit card number is an international standard, and the first 6 digits are BIN, which is allocated by the international card organization. The first card is VISA, the fifth card is Master, and 622 126 ~ 622925 is unionpay standard card. 6 starts with China UnionPay single currency card (or American discover), 4 starts with the international brand VISA credit card (the world's first credit card organization), 5 starts with the international brand MASTERCARD, and 3 starts with Japan JCB and American Express.
The 7th bit-15 bits are allocated according to their own needs, and 16 bits are parity bits.
Other non-16-digit card numbers are allocated by the bank.