This .NET class wraps the AcGeExternalBoundedSurface ObjectARX class. 

ExternalBoundedSurface class is a representation of a bounded surface defined outside of the geometry library. A logical view of this class is as follows: An instance of ExternalBoundedSurface consists of an instance of ExternalSurface representing the unbounded surface geometry, together with a collection of instances of CurveBoundary, each representing a boundary on the unbounded surface. 

It is important to realize that the representation is geometric. No topological information is represented in this class. This assertion is made explicit by deriving this class from Surface. A typical use of ExternalBoundedSurface would be to represent an ACIS FACE (that is, faces on Designer and Autosurf models) as a bounded surface. 

Clearly, an instance of ExternalBoundedSurface can be used as any other Surface. Operations that are well defined on bounded surfaces can accommodate the boundaries within the algorithms (an example is closest point). Alternatively, a knowledge of the precise definition of the external entity allows direct access to the external entity in its native form.

Autodesk.AutoCAD.Geometry.Entity3d
    Autodesk.AutoCAD.Geometry.Surface
        Autodesk.AutoCAD.Geometry.ExternalBoundedSurface

Public sealed Class ExternalBoundedSurface
Inherits Surface

public sealed class ExternalBoundedSurface : Surface;