Represents the "CharSet" entry of the submit-form action object.

It is an optional entry defined as part of the PDF 2.0 specification.

It contains a string representing a PDF string object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 236, page 451.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 239, page 548.