Unfamiliar notation in an AoPS paper

Question

Here it is, from this paper:

Proposition 5.1.1. The number of skyline polyominoes of area $A$ and width $w$ is $\left(\!\binom{w}{A-W}\!\right) = \binom{A-1}{w-1}$.

I'm referring to the first expression, not the binomial coefficient. I think it's a Stirling number of some kind, but I'm not familiar enough with them to be sure.

Answer 1

It’s probably a multiset coefficient;

$$\left(\!\!\binom{n}k\!\!\right)=\binom{n+k-1}k=\binom{n+k-1}{n-1}$$

is the number of multisets of cardinality $k$ that can be chosen from a set of $n$ distinct types of object, and the notation has become moderately standard.