Usually used for lighting circuits, the wiring works like this:
The permanent lives and switched lives of the circuit use the single core and earth cable (6241Y if you want to google it).
This run starts from the MCB and loops between the lightswitches to provide a permanent live and earth to the lightswitchs. Another length of 6241Y is then used from the lightswitch to the light fitting to provide a switched live and earth at the lightfitting.
The neutral cable is a double sheathed cable (6181Y with a blue inner sheath) that runs from the CU neutral busbar and from lightfitting to light fitting (there will only be one neutral at the end of the circuit).
It makes it easier to put light fittings up as there are less cables to mess with at the fitting.
