From what I understand it has to do with how horsepower is rated in the commercial chassis over 14K GVWR. I looked on Ford's website, and all of the cab and chassis F series, even the F350, had 7.3 ratings of 335-350 hp. Interestingly, there were 2WD F350s listed with GVWRs of upwards of 16K. Maybe someone more knowledgeable will chime in, but I seem to remember the real-world output of the engines in the commercial chassis is the same as the consumer-marketed pickups, the only difference is due to how the engines are rated.