15.7.5.3 Use Empty Regions to Reserve Device Resources

If you want to preserve the placement of your existing design but plan additional modifications in the future, create empty regions in the areas of the chip where you plan to add additional logic. As you add new logic, remove or resize your empty regions accordingly to fit your new logic. Empty regions placed over I/O pins reserve them for future use as the I/O needs of your design changes. There are some restrictions for using empty regions in this manner. See the Floorplanning ProASIC/ProASIC ^PLUS Devices for Increased Performance application note for more details.