The same as any other 32 or 64bit OS. It has to do with how "wide" the various bands are. For example, the limits of 32bit on memory is 2^32, or ~4.2GB, whereas 64bit is limited to 2^64, or 16.8mil terabytes. Obviously nothing today uses even close to that, but maybe one day. End all, it's just a limit of how much of value X can be used on various pieces of hardware.