lib/formslib.php setAdvanced

You can use MoodleQuickForm's method setAdvanced to specify a form element as an advanced option :

       $mform->addElement('select', 'display', get_string('displaymode', 'choice'), $CHOICE_DISPLAY);
       $mform->setAdvanced('display');

setAdvanced takes a second boolean parameter which defaults to true. True means set this control as an advanced control. False means it is not advanced.

       $mform->addElement('header', 'miscellaneoussettingshdr', get_string('miscellaneoussettings', 'form'));
       $mform->setAdvanced('miscellaneoussettingshdr');

Will set the header element miscellaneoussettingshdr and all the elements it contains to advanced.

If you set any button in a form to be an advanced item then 'show / hide advanced' buttons are shown automatically in the form. Show / hide buttons are currently displayed at the top right of all fieldsets. The show / hide button shows or hides advanced controls using javascript or if no javascript is available then the whole page is submitted and redisplayed with advanced controls hidden or shown.