Represents the "S" entry of the timespan 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 must always be provided and must always take the value "S".

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 289, page 502.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 300, page 628.