Represents the "TR" entry of the soft-mask dictionary object.

It is an optional entry defined as part of the PDF 1.0 specification.

This property may contain one of two different types:.

1) A string representing a PDF name object.

The PDF specification states that this item assumes a value of "Identity" if no value has been provided.

If provided, this item must always take the value "Identity".

2) A FunctionElement.

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.