The zOrder property defines the stacking order of an object. Objects with a higher zOrder are drawn on top of those with lesser zOrders. Normally the zOrder is determined by the order in which objects are defined in the XML file with earlier objects being drawn under later ones but it can also be manipulated using JavaScript at runtime.
Yahoo WidgetにおけるZorderは値が大きい物から前に描画されます。xmlには後から定義したものほど前になります。こんな当たり前のことが何故かまったく記憶できないので書いておきます。