Represents the "N" entry of the appearance 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) An Element.

2) A FormXObjectElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 168, page 389.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 170, page 472.