Represents the "Subtype" entry of the 3d measurement dictionary object.

It is a required entry defined as part of the PDF 1.7 Extension Level 3 specification.

It contains a string representing a PDF name object.

For definitive details see:.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 9.39a, page 62.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 326, page 683.