[QuickForm]: advaned check box question

I was looking at the example of the advanced checkbox included with the package: $form->addElement('advcheckbox', 'iadvChk', array('Advanced checkbox:', 'Unlike standard checkbox, this element <b>has</b> a value<br />when it is not checked.'), 'Check the box', null, array('off', 'on')); In the third element, there is an array, the first part is the label, what does the second element do? I thought perhaps it's the label to the right of the element but that is the fourth main element ... Thanks, Mark