Electrical voltage between two contacts in an electrical socket is the mean value of the sinusoidal waveform. Its unit of measurement is volt [V].
Continental Europe uses a synchronous 400/230V 50Hz power grid. In North America, power is supplied to households at 120V and 60Hz.
Power distribution grids use alternating current (AC). Alternating current is easily transformed to various voltage [V] levels.
The voltage in the power grid fluctuates slightly; in Europe, it stays within ±10% = 207 to 253 V.
In the context of NETIO products
NETIO smart sockets measure the voltage [V] as a single RMS value common for all sockets.