combinations of binary digits for decimal numbers
The mapping ("0000" => '0' decimal, "0001" => 1, "0010" => 2 ... "1001" => '9' decimal) is one possible allocation of binary codes to decimal.

("0000" => 0, "0001" => 1, "0011" => 2, ...) is another, a Gray code.

("1010" => 0, "0010" => 1, "1111" => 2, ...) is another, randomly generated. The mapping doesn't have to be in order, it's just much more convenient that way.

There are 2.9 • 10^10 such mappings from (all the 16 possible 4-bit codes) to (10 decimal digits).