Represents the "SE" entry of the outline item dictionary object.

It is an optional entry defined as part of the PDF 1.3 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: 153, page 369.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 151, page 443.

This entry references the structure element that this outline item corresponds to, linking the outline tree into the tagged PDF structure tree for reflowing readers.