For xfce4-panel 4.19.4. The latest version of this documentation can be found on-line at

I. Fundamentals
Version Information — Information about the panel version in use.
Standard Enumerations — Standard enumerations used by the Xfce Panel.
II. Panel Plugins
XfcePanelPlugin — Interface for panel plugins
Panel Plugin Register Macros — Register panel plugins that are compiled as modules.
GObject Oriented Panel Plugin Registers Macros — Macros to register panel plugins, written as GObjects.
III. Additional Widgets
XfceArrowButton — Toggle button with arrow
XfcePanelImage — Scalable image suitable for panel plugins
IV. Miscelleanous
Convenience Functions — Special purpose widgets and utilities
Commonly used plugin macros — Various macros to make life easier for plugin developers.
Index of all symbols
Index of deprecated symbols
Index of new symbols in 4.20
Index of new symbols in 4.18
Index of new symbols in 4.14
Index of new symbols in 4.10
Index of new symbols in 4.8
Index of new symbols in 4.6
Annotation Glossary