It can be either sensor for closed loop. Running closed loop will target an AFR of 14.7.
Running in Autotune all the time (not practical with a laptop plugged in) will target whatever AFR is in the Target Table. You need a wide band to run in Autotune.
Low RPM, (idle) is controlled by at PID feedback loop coupled to a min/max PWM.
This is not a UTEC with all the open loop/closed loop transition crap.