Represents the "SMask" entry of the graphics state parameter dictionary object.

It is an optional entry defined as part of the PDF 1.4 specification.

This property may contain one of two different types:.

1) A string representing a PDF name object.

2) A SoftMaskElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 58, page 130.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 57, page 164.