Operation steps: first select m elements from n elements, and then sort them. (One * * * has Anm configuration)

The combination formula Cnm means that M elements are taken out from N elements without arrangement (that is, without sorting).

Operation steps: just select m elements from n elements without sorting. (A * * * has Cnm)

For example, two letters are arranged by three letters: A, B and C, and there are six kinds: AB AC BA BC CA CB.

(The alphabetical order is different, it is a different arrangement)

There are only three combinations of letters A, B and C: AB AC BC (only related to the contained elements, regardless of the order, that is, AB and BA are the same combination).