The tool bar allows you organize your zone layers by drag and drop method. As you add new content to your layout new zone layers will be added on top. All layers behind the others will be on the bottom of the toolbar.
Drag and drop the layers to reorder their position in the toolbar.
By default, layers will be numbered (Zone 1, Zone 2; etc.) Images will show the file name. You can change the layer name and click ‘Apply’ to save your changes: