When in strong ambient light, or have changeable ambient lights, the PD received PPG signals need to be picked up from strong noise.
Ambient light cancellation(ALC) includes AC and DC parts, to supress strong stable DC ambilent light and regular changing light AC components.
To get good quality PPG signals for heart rate or SpO2 applications, the ALC is not a nice to have, but a must to have feature for PPG sensor.