The remote wire originates from the head unit ... and its purpose is to turn on the amplifiers and other components and peripherals ... when the head unit is turned on, it gives a signal to attached components to turn on ... when we turn off the head unit or switch off the engine/ power supply to the headunit, the remote cable turns off the peripherals.
In one of the earlier installations, I faced the issue of low voltage in the remote cable due to which the Fusion amp, last one in the series did not get enough power in the remote signal ... this problem was rectified with the installation of a small switch (sorry, the name just vanished from my memory :))