How combination in a 9 digit number?

There are different numbers of combinations for groups of different sizes out of 9:

1 combination of 9 digits

9 combinations of 1 digit and of 8 digits

36 combinations of 2 digits and of 7 digits

84 combinations of 3 digits and of 6 digits

126 combinations of 4 digits and of 5 digits

255 combinations in all.