This performs the operation "n choose r" where expr1 is n and expr2 is r.
The formula used to obtain the result is:(1)
This is often referenced with Pascal's Triangle and binomial expansion as well as many areas of mathematics.
Several (around 3) commands have a similar function or are used in a similar context to this command. Make a bulleted list of them, with links to the other commands' pages. It will often be the case that several commands all link to each other.