P1 is the connection that can only be used for your smart meter. It will monitor your power and gas totals for your entire home.
S0 is a connection that can be used for pulse meters. So if your solar panels provide a S0 compatible pulse output, you can use it to monitor the power production of your panels.
The youless LS120 supports simultaneous connection of P1 and S0 on 1 youless device. If you use the -EL firmware in the youless Homey can read all data from both P1 and S0 without problem. But you have to add it as 2 seperate devices in Homey (P1 and S0). During pair of S0 you can select that you use the S0 as power meter (and not as water meter). After pairing, you can go to the device settings and select that it is a solar panel meter (meaning, it is measuring power production in stead of power usage)
I hope this clears things up🤗