Hi Giang

Thanks for taking a close look at the PressGang Documentation Guide.

I've had a close look at the DocBook usage as well (it's been a while since I reviewed this section) and I think you've got a point with your observation. Long answer below.

I'll post the descriptions of each element below for context (in case anyone else reads this question in the future):

• <guimenu>
  GUIMenu identifies a menu name in a graphical user interface. In particular, this is distinct from a menu item (GUIMenuItem), which is terminal, and a submenu (GUISubmenu), which occurs as a selection from a menu.
• <guimenuitem>
  GUIMenuItem identifies a terminal selection from a menu in a graphical user interface. In particular, this is distinct from a menu (GUIMenu) and a submenu (GUISubmenu). The distinction between a GUIMenuItem and a GUISubmenu is simply whether or not the selection is terminal or leads to an additional submenu.
• <submenu>
  The name of a submenu in a graphical user interface is identified by the GUISubmenu element. A submenu is a menu invoked from another menu that leads either to terminal items (GUIMenuItems) or additional submenus.

"Choose <menuchoice><guimenu>System</guimenu><guimenuitem>Preferences</guimenuitem><guisubmenu>Mouse</guisubmenu></menuchoice> from the main menu bar ..."

This example was from "back in the day" when GNOME 2 was the desktop on fedora. 

Withouth the markup, the Menu transition is System>Preferences>Mouse

System is the top level menu, so <guimenu> is the correct choice there.

Preferences is not classed as a "terminal" menu, because there are menu choices below this level. I would think that Preferences should be tagged as <guisubmenu> in this instance.

Mouse is a terminal menu choice, so I think this should be tagged as <guimenuitem>.

The corrected sentence would change from:

"Choose <menuchoice><guimenu>System</guimenu><guimenuitem>Preferences</guimenuitem><guisubmenu>Mouse</guisubmenu></menuchoice> from the main menu bar ..."

to

Choose <menuchoice><guimenu>System</guimenu><guisubmenu>Preferences</guisubmenu><guimenuitem>Mouse</guimenuitem></menuchoice> from the main menu bar ...

  NOTE In publican this does not break validation, and I can say with 100% certainty that Publican validates to well-formed and valid DocBook standards. So I think while the gui menu child elements should go in a particular order, the order does not break validation.

I will raise a bug and correct this in the guide.