Represents the "Type" entry of the external data 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 "ExData".

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 170, page 391.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 8.21, page 38.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 332, page 695.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 173, page 477.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 9.39g, page 76.