Click the Features step in the Steps panel.
The top and bottom page is displayed. The top and bottom surfaces are extracted from the surfaces, but you can select different points to change the top and bottom heights of the feature.
The Top & Bottom page enables you to set the top and bottom Z levels for a milling feature. You can enter these dimensions or select them using the Pick location button. When you pick the locations, the dialog warps out of the way until you complete your selection.