nCr a selection of items from a set that has distinct members, such that the order of selection does not matter (unlike permutations). - wikipedia / calculator

5 C 3 = 10: 3 elements subsets of 5 elements set

Combinatorial Number system

The combinatorial number system of degree k (for some positive integer k), also referred to as combinadics, or the Macaulay representation of an integer, is a correspondence between natural numbers (taken to include 0) N and k-combinations, represented as strictly decreasing sequences ck > … > c2 > c1 ≥ 0. Since the latter are strings of numbers, one can view this as a kind of numeral system for representing N, although the main utility is representing a k-combination by N rather than the other way around. Distinct numbers correspond to distinct k-combinations, and produce them in lexicographic order

This can be seen as a kind of perfect hash for certain situation.