It is rare phase tuning needs to be used. It is there to adjust the P5’s software to better phase lock loop synchronize with the incoming AC in certain odd circumstances. In these few cases it helps lower the THD.
There is no harm playing with it. So try the various settings if you would like and listen for a difference. Use the setting that sounds bet.
A little bit of technical info: The regenerator syncs with the zero crossings of the incoming voltage. However the peak of the incoming sine wave is usually not perfectly in the middle of the zero crossings, because of the phase of the distortion products. The regenerated wave thus has to be shifted slightly from the input. Typically, the difference is such the PowerPlant can do this on its own. However, the phase setting can be used when the phase difference is extreme.