Defining the Margin and Surface Properties
Left-click points around the model (on gingiva and/or teeth) to define the margin line.
In the surface properties section there are 3 sliders which allow you to define the thickness of the different areas and the outer smoothing of the splint.
 Occlusal- The thickness of the occlusal surface of the model
 Peripheral- The thickness on the axial walls of the model
 Smoothing- Controls the smoothness of the top surfaces of the splint. The larger the number, the smoother it will be.
In case the margin line covers teeth and palate it is mandatory to check the box  Allow design on palate.
After the parameters are set click Apply.
Defining the Posterior Area
You can flatten the posterior area of the bite splint by clicking two points on the splint where the posterior region starts,
setting the Desired impression depth , and clicking Flatten posterior area 
The next set will take you to "freeform the bite splint top" which is similar to free-forming, see Freeforming "Free" tab.