This is a simple model 50 x 50 x 1 mm. Print at 0.2 mm layer height and you have 5 stacked layers for calibrating your printer on the fly. Stacking layers when adjusting z offset can be useful as the first layer might look good, but when you get to layer 2 or 3 you see that you are a bit off. This test lets you fine tune on stacked layers.