In the simplest case the appearance can be defined by a single GrapeCity.Documents.Pdf.Graphics.FormXObject specified by the Default property.
In more complex cases it can be defined as a subdictionary specified by the Item property. For example, an annotation representing an interactive check box might have two appearance states named On and Off.
See PDF specification for details.
System.Object
GrapeCity.Documents.Pdf.Annotations.Appearance