They don't have the Chassis Cabs out for the 3rd gens yet, so they are offering the box delete as an alternative.
On the 2nd gens, they offered CC's in both 2500 and 3500, but the 2500 had the same wheelbase and rear axle as the pickup. But, the wheelbase of the 3500 CC's is 4" longer than a 3500 dually pickup. The extra 4" gives you a 60" cab-to-axle dimension (an industry standard) as opposed to 56" in the pickups. Thus, 9' utility boxes or flatbeds are fitted to the CC's. Also, the rear axle is about 6" narrower, so the utility bed/flatbed doesn't have to stick out so far to cover the outermost tire. There was also a 2' longer wheelbase offered to fit 11' bodies.
Scott