I see many examples of its usage in the forum but, as far as I can see, it's not documented. Furthermore, the behavior differs between AutoHotkey v1 and v2.
The following statement might be combined with the (expression) entry:
For the AutoHotkey v2 documentation page, the statement could be:For a multi-statement expression, the result of the first statement is returned. For example, (a := 1, b := 2, c := 3) returns 1.
Note that in the v2 documentation, there is an indirect reference to this behavior in the following line for the comma operator:For a multi-statement expression, the result of the last statement is returned. For example, (a := 1, b := 2, c := 3) returns 3.
https://lexikos.github.io/v2/docs/Variables.htm#comma wrote:To include a multi-statement expression in a parameter list, enclose it in an extra set of parentheses. For example, MyFn((x, y)) evaluates both x and y but passes y as the first and only parameter of MyFn.