PI is one aspect of PPG signal quality. It is defined by AC/DC of PPG signal.
The PI value varies for different subjects, diffent optical designs.
Both HR and SpO2 algorithm likes higher PI PPG signal, which means the PPG signal is more high quality, more clear.
However, if PI is good enough, saying over 0.05%, it is no longer an issue for HR algorithm. Things are different for SpO2 algorithm. SpO2 algorithm needs not only higher PI PPG signals. The higher, the better. SpO2 algorithm also need stable PI PPG signals. This is very important for stable and accurate SpO2 numbers output.