How does HFSS Calculate S Parameters?

HFSS says it's "Solved Magnitude" is 1 Watt of Power...

But is it driven by a unit current or voltage (1 Amp or 1 Volt) and scaled afterwords to 1W of power?

And if so, is it the current or the voltage driven by zero phase?

I think HFSS means when you plot the field distribution inside the structure ,the filed strength will be scale to 1 watt input power. just like you placed a signal generator before the input.