You can drag pages from your flatplan onto the holding area. You can open and close the holding area by clicking on the the icon in the top right of your browser.
This holding area is a great way to store some pages that you don't yet know where to place.
The pages that are in the holding area, they still count toward your total page count. You will see that you will either have a gap left on your flatplan, or the page numbering seems to account for too many pages. This is because at some stage the pages on the Holding Area do need to be placed back onto the flatplan.