Let me explain by example.
Q: Given four possible values, {1,2,3,4}, how many 2 value permutations are there ?
A: 16.
(1,1), (1,2), (1,3), (1,4)
(2,1), (2,2), (2,3), (2,4)
(3,1), (3,2), (3,3), (3,4)
(4,1), (4,2), (4,3), (4,4)
However, running 4P2 through Wolfram Alpha gives me an answer of 12.
Q: Similarly, given four possible values, {1,2,3,4}, how many 2 value combinations are there?
A: 10.
(1,1), (1,2), (1,3), (1,4)
(2,2), (2,3), (2,4)
(3,3), (3,4)
(4,4)
However, running 4C2 through Wolfram Alpha gives me an answer of 6.
I'm assuming because the implementations of nCm and nPm removed the element from the source set so it can't be chosen again. (similar to lottery balls picked from a drum).
Is there other terminology/formulae for the equivalent where it's possible to return each value. The real life situation would be dice-rolls, where all 6 options are still available on each subsequent roll of the dice.