This .Net class wraps AcDbUnderlayReference ObjectARX class. 

UnderlayReference is an abstract class that represents underlays in the drawing. Underlays are similar to raster images, but their content is snappable. An UnderlayReference object must reference a compatible UnderlayDefinition object. 

The UnderlayReference object is responsible for the placement of the content within the drawing, while the UnderlayDefinition object handles the linkage to the underlay content. Instances of UnderlayReference-derived concrete classes are inserted into a block table record. Instances of UnderlayDefinition-derived concrete classes are inserted into a dictionary under the named object dictionary.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.Entity
        Autodesk.AutoCAD.DatabaseServices.UnderlayReference
            Autodesk.AutoCAD.DatabaseServices.DgnReference
            Autodesk.AutoCAD.DatabaseServices.DwfReference
            Autodesk.AutoCAD.DatabaseServices.PdfReference

Public abstract Class UnderlayReference
Inherits Entity

public abstract class UnderlayReference : Entity;