Represents the "P" entry of the structure element dictionary object.

It is a required entry defined as part of the PDF 1.0 specification.

This property may contain one of two different types:.

1) A StructureElementElement.

2) A StructureTreeRootElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 323, page 558.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 355, page 721.