Originally, the Stages were a way for ROUSH to allow customers to choose varying trim levels.
This
site listing can highlight all the trim levels.
Since ROUSH has gone to the centralized production route they now offer a lot of trim levels at dealerships so customer now have to find a car they want on the lot. There is always the option to buy a
clean slate and send it to ROUSH for your desired trim level.

The Stage 1 is the show without the go!
The Stage 2 is all the show and some handling go!
Obviously, the Stage 3 has all the catalogue thrown at it .... show, go and handling.