This .NET class wraps the AcGeExternalCurve2d ObjectARX class. 

ExternalCurve2d class is a representation for a 2D curve geometry that has its definition external to GeLib. Since ExternalCurve2d is a Curve2d, these external entities can be treated as any other GeLib 2D curves. Certain external curve points sets may have corresponding native representations in GeLib. In such cases, it is possible to obtain such GeLib representations. However, the curve definitions of native GeLib curves would differ from the definition of external entities. An example of such a difference is differing curve parameterizations. A typical example of a use of ExternalCurve2d would be to represent a ShapeManager pcurve. 

Autodesk.AutoCAD.Geometry.Entity2d
    Autodesk.AutoCAD.Geometry.Curve2d
        Autodesk.AutoCAD.Geometry.ExternalCurve2d

Public sealed Class ExternalCurve2d
Inherits Curve2d

public sealed class ExternalCurve2d : Curve2d;