This class represents the projection annotation. This is definitively detailed in:.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: page39, page 39.

This class is always an indirect object because D3DAngularMeasurementElement.EntryS, D3DCommentNoteElement.EntryS, D3DLinearMeasurementElement.EntryS, D3DPerpendicularMeasurementElement.EntryS and D3DRadialMeasurementElement.EntryS require it to be so.

System.Object
    WebSupergoo.ABCpdf13.Elements.Element
       WebSupergoo.ABCpdf13.Elements.AnnotationElement
          WebSupergoo.ABCpdf13.Elements.MarkupAnnotationElement
             WebSupergoo.ABCpdf13.Elements.ProjectionAnnotationElement