This class represents the thread action. This is definitively detailed in:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 205, page 423.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 209, page 514.

System.Object
    WebSupergoo.ABCpdf14.Elements.Element
       WebSupergoo.ABCpdf14.Elements.ActionElement
          WebSupergoo.ABCpdf14.Elements.ThreadActionElement

A thread action navigates to a specific article thread and optionally to a specific bead within that thread. The F entry identifies the thread by index or title string. The B entry specifies a starting bead.