According to 9.1.4.B
"GCR listed IT cars, 1985 and newer, under their current IT
specifications shall compete in STU."

Let's just say you wanted to run ST in your IT car, and your car has mods not allowed in IT (missing washer bottle or poly engine mounts for example), would you then be required to make other changes to the car that are required in ST, but but not in IT?

One example of what I am thinking about is the fire system requirement. In IT, you don't have to have a fire system , but in ST you do. (9.3.23. FIRE SYSTEM All cars shall be equipped with an On-Board Fire System except Showroom Stock, Touring, Spec Miata, and Improved Touring.)

So if I understand this correctly, if I make a change that isn't allowed in IT, but is in ST, and I run in ST, I have to comply with all rules that apply to ST and I lose the IT crossover allowance.