About Positioning the Mesh Elements
You can line up or position the mesh elements of the control mesh with reference to a datum plane or a view orientation to refine the Freestyle surface. In case of a datum plane reference, you can either select a datum plane or use the plane that is automatically selected. The reference plane is selected automatically based on the following calculations:
• If you select faces, the corresponding vertices are projected onto a least squares plane. This plane is calculated based on all the selected faces.
• If you select edges, the corresponding vertices are projected onto a calculated plane to form a straight line or planar loop.