Options packages. Generally the Deluxe was the base model and the Premium was pretty well loaded.
This was available from 2001-2004. What came in the standard features did change somewhat by year but for the most part the premiums got leather, abs/tc, Mach 460 and better wheels as a standard feature.
Deluxe cars got cloth, base radio, no abs but those things could be ordered as individual options. The only thing you couldn’t upgrade was wheels.
so for the GT deluxe, for 2001-2004 you got the painted 5-star 17” wheels, and the 01-04 GT premiums got the bullitt wheels. The 03/04 also had a bright wheel option that was not available for the deluxe I believe.
Unsure if the VIN will tell you. I have the window sticker of the ‘03 GT premium I bought new and can take a peek to see if it says Premium on it.