Represents the "Subtype" entry of the optional content usage print dictionary object.

It is an optional entry defined as part of the PDF 1.0 specification.

It contains a string representing a PDF name object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 102, page 232.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 100, page 284.