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

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 333, page 696.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 9.49, page 77.

This class is always an indirect object because RichMediaExecuteActionElement.EntryTA requires it to be so.

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

A rich media annotation embeds interactive media content - such as Flash, video, or a 3D scene - within a rectangular area of a PDF page.

The RichMediaContent entry holds the assets, configurations, and views. The RichMediaSettings entry holds the activation and deactivation behaviour.