Represents the "Subtype" entry of the 3d background 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.

The PDF specification states that this item assumes a value of "SC" if no value has been provided.

If provided, this item must always take the value "SC".

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 306, page 527.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 317, page 659.