I think this is ultimately in the eye of the beholder. In general terms I would say almost anything but fire alarms and "power" circuits. It is hard to limit this by voltage or current. A garden variety POTS phone is over 100 volts when it is ringing and speaker wires in a concert situation will have lots of amps (peak current).
You also see lots of volts and amps in a radio transmitter but the FCC licenses those guys.
