Represents the "USm" entry of the 3d units dictionary object.

It is an optional entry defined as part of the PDF 1.7 Extension Level 3 specification.

It contains a double representing a PDF numeric object.

For definitive details see:.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 9.33a, page 60.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 325, page 680.