This class represents the user properties attributes. This is definitively detailed in:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 328, page 569.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 361, page 736.

A user properties attributes dictionary carries an array of user property dictionaries, each defining one custom property of a structure element. User properties allow applications to attach arbitrary named data to structure elements beyond the standard attribute sets.

The P entry holds the array of individual user property dictionaries. Each defines one property with a name, value, formatted display string, and a visibility flag.

System.Object
    WebSupergoo.ABCpdf14.Elements.Element
       WebSupergoo.ABCpdf14.Elements.AttributesElement
          WebSupergoo.ABCpdf14.Elements.UserPropertiesAttributesElement