Switched neutral option in switch datasheet
Left goes to mains, middle to your appliance, right goes to ground (maybe via a resistor??). The difference between 5503 and 5508 is that 5503 disconnects both sides of the lamp when off, while 5508 keeps the ground side connected. So the '(un)switched neutral' refers to the neutral of the lamp only.

I don't see the purpose of the 'disconnect the lamp's neutral': the other side of the lamp is still connected to the neutral via the load, so for safety purposes it is still connected.