Determine whether the following problem involves a permutation or combination.​ (It is not necessary to solve the​ problem.)
How many different ​-letter passwords can be formed from the letters ​S, T​, U​, ​W, X​, Y​, and Z if no repetition of letters is​ allowed?

The problem involves (combination or permiation) because the (order or number) of letters selected (does or does not) matter.