The 12 volt option adds the 3.3 and 5 volt regulators needed to power the pic and '485 chips, respectively, from a stand alone 12 volt DC source.
If using a pc power supply, it provides all of the needed voltages without the extra parts. PC power supplies are the least expensive power supplies on a dollar per watt basis.
If using a pc power supply, you should not install the other regulators.
It is also important to look for a power supply with a single 12 volt power rail. Multiple rail models could cause problems depending on how that particular power supply was designed.