Setting a Scale Value to Restricted assigns a value to that cell in the output weighted overlay result that is the minimum value of the evaluation scale set minus 1 (zero in this exercise). If there are no inputs to the Weighted Overlay tool with cells of NoData, you could use NoData as the scale value to exclude certain values. However, if you have NoData cells in any of your inputs, it is safest to use Restricted. Potentially, a result from the Weighted Overlay tool could contain cells of NoData that have come from one or more of the inputs (NoData on any input equals NoData in the result) and restricted areas that you intentionally excluded. NoData and Restricted values should not be confused. Each serves a specific purpose. There may be areas of NoData for which you don't know the value but that are actually suitable areas. If you use NoData to exclude certain cell values and there is NoData in one or more inputs, you will not know whether a cell of NoData means the area is restricted from use or there was no input data available in that location.