What are "united inched" in window measurements?

United Inches (http://www.marthabeth.com/united_inches.html)

United Inches Calculator (http://www.baxtersframeworks.com/unitedinchescalc.html)

It's the sum of the length and width of each window. It's a very crude measure of area, and the only reason it works is because windows tend to have aspect ratios that similar (with lots of exceptions).