Any IT-compliant car (even ITR) can compete in STU.
Any IT-compliant car 2L displacement and less can compete in STL.
In either case, you must completely conform to the IT regs, including weight, in which case you must display your IT weight (though you must also display the STU or STL class.) However, if you deviate in any way* from the IT regs, you must completely conform to the STU/L regs, in which case you'll display your STU/L weight.
GA
* A good example: any SM car can also compete in STL. However, we're getting rumors that some 1.8L SMs think it's OK to pull their restrictor plate and still compete in STL as an SM. That is incorrect; to meet the regs to compete in STL as an SM, they must "completely conform...to Spec Miata class specifications". If they happen to otherwise meet the STL specs they're fine, but many do not meet the 5" ride height requirement and many of the allowed SM head mods do not strictly meet the STL regs (as they do not in ITA/S).