It's z-order, i.e. the order in which the windows are stacked on top of each other. It's usually the same as the order in which windows were last used, but with minimized windows at the end.

What determines the order of window tiles in the Windows Catalog view?

I switch between 2-3 different programs often out of 7-10 total and normally it seems to list in the order last used, but sometimes I see my previous program used showing up at the end making me hunt for it.