There are two versions: there is a thread in one that can thread into your factory o2 bung and be turned 180 degrees to clear the car, or there is a weld in version.
This will remove the o2 sensor from the direct exhaust flow path and reduce the chance of throwing a Check Engine Light for installing a high flow cat.
By running the rear o2 this will keep the car from running the catalyst test mode and reducing your highway MPGs.