What a field is: a non-terminal field or one of the terminal field kinds, from its type and flags.
public enum PdfFieldKind
ISO 32000-2 §12.7.2 (terminal and non-terminal fields), §12.7.5.2 Table 229 (Pushbutton and Radio decide the button kind) and §12.7.5.4 Table 233 (Combo decides the choice kind).
kind:method, kind:property, kind:ctor. Press Esc to clear.CheckBoxA check box (PdfCheckBoxField).ComboBoxA combo box (PdfComboBoxField).ListBoxA scrollable list box (PdfListBoxField).NonTerminalA field whose children are fields (PdfNonTerminalField).PushButtonA push button (PdfPushButtonField).RadioButtonA set of radio buttons (PdfRadioButtonField).SignatureA signature field (PdfSignatureField).TextA text field (PdfTextField).UnknownA terminal field whose type is missing or unknown (PdfUnknownField).