The Drive Surfaces are a selection of one or more surfaces upon which the toolpath is created. Most important for the selection is the normal direction of the surfaces/faces. Since a surface has two sides, the toolpath is determined to be located on the front side of the surface. The front side is the side where the surface normal shows out. For solids, the normal direction is usually easily defined, it points outside of the model. An exception is when a part file is imported or converted and the surface normal directions change. Changing the surface normal direction is accomplished using the Utilities menu Reverse Surface Normal function.
Reversing the surface normal direction for multiaxis drive surfaces is done during geometry selection, as explained in the Selection Manager.
TIP: To show the surface normals, in the View menu, click Surface Normal. You can also use the keyboard shortcut N to toggle the visibility of the surface normal (click anywhere in the Workspace to give it key focus first).
When preparing to create Multiaxis toolpaths, you must check the surface normal direction of the part to assure that the toolpath is created on the proper side of the surfaces/faces.
1 To show the surface normal direction, in the View menu, click Surface Normal.
You can also press N to hide or show the surface normal.
The following image shows a part that has three faces with incorrect normal directions. (These are the floor face, the vertical wall face, and the face on the right-end of the part.)
2 To change the normal direction, in the Utilities menu, click Reverse Surface Normal.
3 In the Workspace, click to select each face/surface that has an incorrect normal direction.
4 To confirm your selections,
right-click anywhere in the Workspace,
and click 
OK.
You can also press Spacebar.
When you confirm the selections, the normal direction is reversed for each selected face.
Notice the new direction of the normal for each of the selected surfaces compared to the original directions. The surface normal must show (point out and away) from the side that is being machined.
5 To cancel the function, in the Data Entry tab, click Cancel.
This concludes the example.