iFit Standard threads are for people who want to use the iFit bore system in their current barrel. These do not change the threads like some iFit adapters do. The iFit barrel boring system is an amazing way to improve your markers accuracy, because your barrel will always be the proper bore size for the paint you are shooting.
The TPX version is only for use with the TechT iFit barrel, and will not work with any other barrel.