Drive surfaces are the model geometry which is used for tool path generation; it is the surface you want to work on and defines the shape to be milled.  You can select multi surfaces. Drive surfaces determine the tool location and tool axis in some cases.

This section allows you to select the surfaces which act as the drive surfaces.

Select Surface(s)  

Selecting this button minimizes the dialog and prompts you to select surfaces. To end the selection press the enter key on the keyboard or right mouse button click.
 
The selected surfaces are displayed under Selected Machining Feature(s) list. Selecting a surface from the list highlights the geometry on the graphics area and displays the surface normal.

Drive Surfaces(s) List

Drive Surface, highlighted

Remove All  

Removes all Selected Surfaces from the Selected Machining Feature(s) list.

Remove Active

This allows you to remove a region from the selection list. You need to select a region from the list of Selected Machining Feature(s) and click Remove Active or press the delete key on the keyboard.

Surface Normals

Tool path orientation is determined by surface normal direction. So if you have generated the tool path on the wrong side you can simply change the surface normal by flipping the surface normal and regenerate your tool path again. The surface normal for each surface is represented by a green arrow on the surface.

Flip All Normals  

Selecting Flip All flips the surface normals of all surfaces selected as Machining Features.

Flip Active Normal

Selecting Flip Active flips the surface normal of the selected surface under Machining Region.

Flip Active Normal