wp_nav_menu-菜单显示优先

Given a theme_location parameter, the function displays the menu assigned to that location.

If no such location exists or no menu is assigned to it, the parameter fallback_cb will determine what is displayed.

If not given a theme_location parameter, the function displays

the menu matching the ID, slug, or name given by the menu parameter;
otherwise, the first non-empty menu;
otherwise (or if the menu given by menu is empty), output of the function given by the fallback_cb parameter (wp_page_menu(), by default);
otherwise nothing.

优先级:theme_location >