check the manual, but i'd be willing to bet that the SW is for the subwoofer output. if you have subwoofer controls on the head unit (part of how it's programmed) then chances are very good that those SW lines are specifically for the subwoofer. the rear output can be used for anything really. i've had to deal with head units that only had one output, so i've used them for front/rear/sub and had no problems.

i konw personally i'm hesitant of head units with that stuff, as i like to run my subs between 90-100hz instead of straight cut off at 80hz. brings the bass lines in the music i listen to out just a little bit more. i also cut my front speakers at 70-80, depending on how good they are. i don't want/need a head unit or any other piece of equipment making these decisions for me. i'd rather wire in an active crossover (which i did in my last car).