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

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 190, page 411.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 193, page 498.

This class is always an indirect object because it inherits from AnnotationElement which is always an indirect object.

System.Object
    WebSupergoo.ABCpdf14.Elements.Element
       WebSupergoo.ABCpdf14.Elements.AnnotationElement
          WebSupergoo.ABCpdf14.Elements.WatermarkAnnotationElement

A watermark annotation displays a graphic at a fixed physical size and position when printed, regardless of scaling applied to other page content.

The FixedPrint entry references the fixed print dictionary that holds the transformation and page-fraction anchors.