When you enable the Facing pages check box, the content on the facing pages is merged on one page. The layer structure for the merged page is based on the left-hand page. The layers from the right-hand page are inserted above the layers on the left-hand page. This rule also applies if you reorder the pages. If you disable the Facing pages check box, the layers and content are redistributed across separate pages. Objects that cross both pages are assigned to a page according to where the center of the object is found.