To build and use surfaces in FeatureMILL, you need to understand how FeatureMILL defines a surface. A surface is defined by a rectangular set of points and a description of how the surface behaves at each point on the surface which determines the smoothness of the surface. The set of points determines the shape, or geometry, of the surface.
The rectangular nature of the control point mesh means that a surface has four boundary curves. In some cases, surfaces are constructed where one or two (opposite) boundary curves are degenerate or form a single point such as the poles of a sphere. In other cases where surfaces wrap around, such as a cylinder, two opposite boundaries can be the same curve and are called seams. With seams and degeneracies, a surface may appear to have only three, or two, or even no boundaries (a sphere), but the four boundaries are always defined. With these four boundaries, you can break a surface into rows and columns so surfaces have a table-like structure.