When you select it, the blank white page on the right will now have a
dotted line. The dotted line represents where we (the printer) will give you when we're completely done. When we print, we need that extra space outside the dotted line. If we don't have that extra art, then you will likely see thin white lines when your final print is made. This happens because every sheet we print moves around a little bit even when we're trying hard to print perfectly.