AFR Sensor and O2 Sensor - Primary / Secondary Question

Hi guys,

Quick question.

How do I know which one is the primary and secondary? I am thinking of changing them both soon but unsure which is which.

I understand there is the downstream / upstream term.

Is the one closer to the engine the primary upstream (Before catalytic converter) and the one after, closer to the exhaust exit the secondary downstream?

Also, just want to confirm - is the primary one known as the AFR sensor and the secondary one known as the O2 sensor?

I know this is such a noob question but I could not really find a thread on it even with Google's help...

Thanks for checking out my thread, any guidance greatly appreciated~!

AFR Sensor and O2 Sensor - Primary / Secondary Question