My manual says that light electric is for trailer up to 10,000, heavy electric is for trailers over 10,000 pounds. The hydralic light and heavy read the same.
As I remember when messing with the light and heavy electric settings all it seemed to do was change the gain. If I choose light I think it set the gain at 2 or 3, heavy set the gain at 5ish. I kind of think the truck remembers where you left the gain for each of the four settings so if you tow a light trailer and use a gain of 4 then tow a heavy with a gain of say 7 all you have to do is switch from light to heavy electric and your truck will remember what gain you like.
I couldn't tell you if I have mine set on heavy or light but I do know my gain is set on 6 to 6.5 for freeway driving and I set it to 5 when driving the city streets. I like to have my brakes set tight when cruising down the road.
