Its basically what the factory did to the truck different between the previous model. Job 1 trucks where the first ones out, and they had some problems with leaking radiators, and bad fuel economy. Job 2 trucks came out with the lower valance moved forward for areodynamic reasons, and on average, if you put that valance on a Job 1 truck, you can get a good 1-2 mpg increase. Job 2 trucks also came with a back up camera feature, I believe, and it tells you when your in a regen. Job 3 trucks are basically all the same as Job 2 trucks as far as I can tell, except they come standard with a 3.55 gear ratio, instead of a 3.73 standard gear ratio as the Job 1 and 2 trucks had.
Theres probably a lot I have missed, but that is all I have gotten out of it.