I've also understood the designation to refer to GVWR and such other ratings. If your truck is listed with a GVWR of 8,800lbs, it's a Super Duty. My Ford dealer told me that Super Duty was a designation Ford has used at various times throughout the years, mostly because it sounds good, and it delineates a truck built to handle a larger payload. So according to him, if it's got the higher GVWR, it's a SD.

If I've got this wrong, somebody please let me know.

Rotorhead