Defining Faces Option
Defining Faces, the second option, prompts you to select two opposing faces to define a midsurface for the face pair. If you chose two opposing faces, the midsurface is generated from the underlying surfaces midway between the faces. The defining faces may have any type of underlying surface, and do not have to be the same type.
The system also looks at the defining face types to determine an optimal geometry type for the midsurface geometry. If a defining face contains a B-surface face, the resulting midsurface type is a B-surface. These B-surfaces are generated by averaging the surfaces at corresponding parameter points.
