A rectangular surface defined by a grid of points that represent the control points of a Non-Uniform Rational B-Spline (NURBS) definition of a surface.
NURBS is a common mathematical representation in the CAD industry.
NURBS surfaces are not often used because they are limited to a rectangular shape; trimmed surfaces are more useful.
Surfaces provide important information for meshing operations, producing better meshes than facet-based mesh input can achieve.