I can't help you about the PNs but both O2 sensors are used to regulate in a sense. The main purpose of the second O2 sensor is to check the cat convertors efficiency (making sure that it's working). The front O2 sensor does most of the work.

Now, why are you replacing O2 sensors? If you're seeing codes for both, double check all vacuum lines as that'd be the cause, not 2 bad O2 sensors at the same time. You'ld be surprised how many times I've seen that misdiagnosed for what shouldn't have cost more than some diagnostic time plus a little vacuum hose.