Represents the "D" entry of the media clip data dictionary object.

It is a required entry defined as part of the PDF 1.0 specification.

This property may contain one of two different types:.

1) A FileSpecificationElement.

2) A FormXObjectElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 274, page 492.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 285, page 617.

The D entry holds the media data reference as a file specification dictionary or a URL string. It identifies where the media content is located, either embedded in the PDF or at an external address.