This class represents the combo field. This is definitively detailed in:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 231, page 445.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 234, page 539.

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

System.Object
    WebSupergoo.ABCpdf14.Elements.Element
       WebSupergoo.ABCpdf14.Elements.FieldElement
          WebSupergoo.ABCpdf14.Elements.ComboFieldElement

A combo field presents a drop-down list of options and optionally allows the user to type a value not in the list. The TI (top index) entry records which option is scrolled to the top of the visible list. The I entry records the indices of selected items.