When determining combinations the order doesn’t matter. With permutations, the order of the numbers does matter. To determine number of combinations you can use the following formula:

nCr = n!/r!/(n-r)!
where nCr is the number of combinations
where n is the total number of items that can be chosen
and r is the number of items chosen from the n items available.

! represents the factorial symbol, for example:
7! = 7 x 6 x 5 x 4 x 3 x 2 x 1 = 5040
3! = 3 x 2 x 1 = 6

So in a lottery where you have 99 numbers and you have to choose 6

You can list down a lot of combinations with it if you use these.

regards,
Leika