You can create submenus only if 'Advanced mode' is activated in the default project settings.
Then you have the possibility to add further submenus within the main menu or a titleset menu (also for sub-title selection or language...).
Just select "Add menu" from the buttonbar and click on the menu background. In the following dialog you can set the type of menu: Chapter (ptt), audio, subtitle, angle (yet not tested) or simply submenu (for the main menu only submenu is possible). This setting is only important for the direct start of a menu from your remote, so that exactly this menu is started, if you press the appropriate key on the DVD player remote. Default is "Submenu" thus nothing 'special'. If you need only a simple pre and/or post command you can enter them here, otherwise open the menu background dialog (right click on the menu background) for the specific menu afterwards. In the current menu a button 'next menu' is created then, in the new menu a button 'to return' (Back-Button) is also already created. In a main submenu you may add further titles, in a titleset submenu you can insert chapters (from the chapter list) or special buttons with 'insert button' (e.g. for audio selection, sub-title selection, jump to another menu...). Since there are so many possibilities, it is therefore 'advanced'.