This .NET class wraps the AcGePointOnSurface ObjectARX class. It represents a point on a surface. This class encapsulates all of the geometric information of a point on a surface, including its parameter value, 3D coordinates, derivatives, and normal vector.

Autodesk.AutoCAD.Geometry.Entity3d
    Autodesk.AutoCAD.Geometry.PointEntity3d
        Autodesk.AutoCAD.Geometry.PointOnSurface

Public sealed Class PointOnSurface
Inherits PointEntity3d

public sealed class PointOnSurface : PointEntity3d;

Geometry library documentation in ObjectARX Developer's Guide