Old basically says the cars are 'similar to World Challenge', new describes them without using WC verbiage, as well as describing each individual class prep (since they're more than just three different levels of the same prep). It also clarifies a point of confusion by noting that STO cars are specifically classed where as STU/STL are "catch-all" as long as they meet the basic requirements.

It also specs a new upper displacement limit of 3.2L for STU, so that we can dispense with the ongoing requests for 3.2-ish liter explicit allowances, and also opens the door for "pony car" explicit allowance with some restrictions.

Or, you can just read it for yourself.

GA