For many staircase models, the step height can be adjusted (reduced) by milling. This to achieve the exact height between the two (finished) floors, should there be a difference between the and the height of the staircase.

If, for example, the height between the two floors is 294cm and you want to want to use the Paris model with steps of 20cm each, you'll end up with a height of 300cm with 14 steps and the landing step (15 heights of 20cm), which is 6cm too high. By milling 4mm per step, the height of one step will be 19.6cm and the total height of the staircase will be exactly 294cm.

Note that the number of millimeters per step that can be milled differs depending on the model.