Represents the "Type" entry of the signature reference dictionary object.

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

It contains a string representing a PDF name object.

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

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 253, page 469.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 256, page 568.