tutorial/tutorial4.xxe_gui:

  <preferencesSheet name="countWordsOptions">
    <class>CountWordsOptions</class>
  </preferencesSheet>

  <preferencesSheets name="preferencesSheets">
    <insert />
    <preferencesSheet name="countWordsOptions" />
  </preferencesSheets>

The custom preferences sheet implemented by Java™ class CountWordsOptions (code contained in XXE_install_dir/doc/gui/tutorial/custom_parts.jar) is declared using a preferencesSheet element.

Then this sheet is inserted in the standard set of preferencesSheets (preferencesSheets element called "preferencesSheets" found in DesktopApp.xxe_gui), after all the other sheets, by the means of the <insert/> facility.