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

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

It contains a string representing a PDF name object.

This item may take one of the following valid values:.

• U3D
• PRC

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 300, page 516.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 9.35, page 56.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 311, page 645.