how do you know when to use permutation or combination?

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oh well if it repeats again then use permutation and if it doesnt then use combination.for example how many different words can be formed from the word RANDOMin this case u use permutations as RAN and NAR are both different words.while on the other hand, out of 5 people, how many groups of 3 people can be formed is a combination since Andy, louis , sam & sam, andy, louis is the same group.