I have learned that permutation deals with arrangement and order while combination deals with arrangement but the order is not important.

I understand the definition but when a question comes I can't distinguish which formula to use either permutation or combination :'(

example

in how many ways can 5 starting position on a NBA team be filled with 12 men who can play any of the position?

(What is it and what shall I use? And how to distinguish permutation and combination question really feeling confused ?