The little placement box is in the shape of a diamond.... Now i thought only ISO games did this?
Starcraft uses somekind of dual approach where the world is created using an isometric based tile system. All other objects like trees, buildings, vehicles, soldiers, are using a square based system. They are placed and move accoording to that grid as well. This method of working has a few advantages. First is of course the 3d-ish look and feel that you can't have with a square tiles (well you can actually, but it's a lot harder to do). Second, because all other sprites do use the square based approach, calculations for, for example, pathfinding become so much simpler.