Click on create constraints to start drawing your constraint in the 3D view. Two modes are available for the plane definition:

• Average: The plane used by the reconstruction will be averaged on all the polygon vertices clicked in the 3D view.
• Horizontal: The plane used by the reconstruction will be horizontal and averaged on the vertices height mean value.

Press enter or close the polygon to define the constraint. Use the arrows to modify the elevation of the constraint if needed.