Represents the "TransferFunction" entry of the type 16 halftone 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.

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: 133, page 314.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 131, page 377.