Shelly 3EM 3-phase meter accuracy

Good morning

Have a Shelly 3EM 3-phase meter here waiting for the electrician to be installed…but have two question regarding this model and I don’t want just for this subscribe to some other forum (o;

  • What is the 4th current sensor for which goes over the neutral line?

  • How accurate are the power readings when a 3-phase device is connected without neutral compared to a normal device with just using one phase and neutral?

thanks in advance