BCD code uses 4 binary digits to represent a decimal number. The 8421 code is usually used (there are others such as 2421 code and remainder 3 code).

Let's use the 8421 code to explain it here.

Four-digit binary system. Corresponding to the numbers 8, 4, 2, and 1.

For example: 1010, then the corresponding number is 8 0 2 1, which together equals 11.

For example: 0101, then the corresponding number is 0 4 0 1, which adds up to 5.

Same reason

The code 2421 corresponds to the four numbers 2, 4, 2, and 1.

The remainder 3 digits are 8421, but after using them, you need to subtract 3.