Represents the "AC" entry of the appearance characteristics dictionary object.

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

It contains a string representing a PDF string object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 189, page 409.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 192, page 496.

The down caption replaces CA while the pointer button is held down over the widget. If absent the normal caption CA is shown in the down state.