15.7.5.1 Use Empty Regions to Guide the Place-and-Route Process

If your design does not completely use up your target device (for example 60% utilization or lower), use empty regions to cluster your logic placement into specific subareas of the chip. This helps when you have originally placed-and-routed the design into a smaller device but want to fit it to a larger part while still preserving the performance you have achieved in the smaller device.