Represents the "S" entry of the soft-mask 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 may take one of the following valid values:.

• Alpha
• Luminosity

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 144, page 347.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 142, page 416.