I saw an example in another forum that used "--" like this:

=SUMPRODUCT(--($A$2:$A$9=$J$2),--($B$2:$B$9=$K$2),(D$2:D$9))

What does qualifying the array expression with "--" do?

In this case the arrays in the first two expressions (columns A and B) contain text. The formula is seeking a match for the two text columns then returning another column (D) with corresponding data.

I do not have a specific problem to solve but am wondering about this usage.