This .NET class wraps the AcGeExternalSurface ObjectARX class. 

ExternalSurface class is a representation for an unbounded (other than by the parametric bounds of the surface) surface geometry that has its definition external to GeLib. Since ExternalSurface is a Surface, these external entities can be treated as any other GeLib surface. Certain external surface points sets may have corresponding native representations in GeLib. In such cases, it is possible to obtain such GeLib representations. However, the surface definitions of native GeLib surfaces differ from the definition of external entities. An example of such a difference is differing surface parameterizations. A typical use of ExternalSurface is to represent a ShapeManager surface. 

Clearly, an instance of ExternalSurface can be used as any other Surface. Alternatively, a knowledge of the precise definition of the external entity allows direct access to the external entity in its native form. This access is more efficient since it avoids the level of indirection that is incurred when accessing the external entity through GeLib interface.

Autodesk.AutoCAD.Geometry.Entity3d
    Autodesk.AutoCAD.Geometry.Surface
        Autodesk.AutoCAD.Geometry.ExternalSurface

Public sealed Class ExternalSurface
Inherits Surface

public sealed class ExternalSurface : Surface;