AC voltage / ATM90E26 reference circuit confusion

[cmorgan]

I am trying to design a circuit to measure power consumption with the help of ATM90E26 Application note http://ww1.microchip.com/downloads/en/AppNotes/Atmel-46102-SE-M90E26-ApplicationNote.pdf Looking at page 4 it looks like they are considering the L line to be the ground for this section of the circuit. They also have a comment in the introduction where they say "Considering low-cost and anti-tampering etc., ground of the single-phase energy metering IC is generally connected to the phase line (L line) of the single phase power supply."

I'm aware of the issues of connecting directly to mains like this and plan to use an isolation IC to separate this portion from the rest of the circuit.

Is it typical to use one of the hot legs, the L line, as a circuit ground? Why not the other way around?

What if I'm using two hot legs of 240VAC and no neutral? Can I just hook them up interchangeably using one of the legs as the circuit ground?

I've done mostly digital electronics and I'm having trouble conceptualizing what occurs if the circuit ground is an AC signal like in this circuit. Doesn't the potential across the capacitors change as the AC voltage changes in that case? Or are the grounds indicated on the page 4 schematic not all the same voltage reference even though they are the same symbol?

Chris