This page of the wizard allows you to restrict machining of the model to that area of the model which can be machined by the tool. This avoids creating vertical walls around the edge of the model where the model starts curving back underneath and hence the tool would not be able to reach.

The cut plane can also be used to easily create a 3D model by cutting it in two sections. First cut the front of the model with the cut plane positioned in the centre, then cut the back of the model - again with the cut plane in the centre, and assemble the two sections to form the complete 3D object.

The cut plane can be positioned simply by clicking in the image or centred or set to a specific height, you decide.