Surfaces can be extended beyond the edges created by their defining objects using the Extend setting of Surface Properties. When the extend equally in all directions option is checked all edges of a surface will be extended. If this is unchecked each edge of a surface can be extended individually. In some cases the extension can not be calculated or the extension can cause the surface to become invalid. When you apply an extension to an existing 3D surface if the surface can no longer calculate its 3D position a warning dialog will appear.
Path surfaces in polygon form and silhouettes cannot be extended.
Note: applying a negative extension will shrink the surface.