The PointCloudDefEx object (or "point cloud extension definition object") controls the actual point cloud data associated with one or more PointCloudEx entities (or "point cloud extension entity"). These objects link the .rcp/.rcs file to the dwg file, similar to how Xrefs work. 

The relationship between the PointCloudDefEx and PointCloudEx classes is much like the relationship between an AutoCAD block definition object and a block reference entity. 

PointCloudDefEx objects are stored in a special db dictionary named "ACAD_POINTCLOUD_EX_DICT". These objects are not AutoCAD entities, so they cannot be displayed or selected by the end user.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.PointCloudDefEx

Public Class PointCloudDefEx
Inherits DBObject

public class PointCloudDefEx : DBObject;