An annotation of a type Table 171 does not list, or without a subtype: its common entries only.
public sealed class PdfUnknownAnnotation : PdfAnnotation
ISO 32000-2 §12.5.2 and §12.5.6.1: the set of subtypes is extensible. Subtype gives the raw name; a processor without a handler for it displays the normal appearance unless the Invisible flag is set (§12.5.3).