Hit the "Advanced" button to enter the advanced option window.
When you activate "generate toolpath only at front side" then toolpath will be generated only between the two surfaces. It is the side the surface normal points from.
The default toolpath would look like this. You see a simple impeller section with two blades. toolpath is all around the blades, even on the backsides.
With the option activated the toolpath is only between the first and second surface. What happens is that the two enclosing surfaces will be extended virtually till the end of the drive surface. The red surfaces are the real surfaces. The yellow ones are the virtual extended ones.
The first and second "surface toolpath tangent angle" limits the toolpath generation. Imaging that you tilt the virtual extended surfaces with an angle. You can set it for the first and for the second surface.
