tutorial/tutorial2.xxe_gui:

  <action name="aboutAction" label="_About This Document Editor">
    <class>AboutAction</class>
  </action>

It is a custom action implemented by Java™ class AboutAction (code contained in XXE_install_dir/doc/gui/tutorial/custom_parts.jar) which displays our custom About dialog box.

Naming our action "aboutAction", just like the standard action "aboutAction" found in DesktopApp.xxe_gui, suffice to do the job.