Represents the "U" entry of the number format dictionary object.

It is a required entry defined as part of the PDF 1.0 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: 263, page 482.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 268, page 595.