The ceiling rose, sometimes called a ‘loop-in ceiling rose’ or ‘3 plate ceiling rose’ is effectively a junction box for the power feed, switch wire and pendant flex of a ceiling light:
The ceiling rose has 3 sets of discreet terminals for the current carrying conductors plus an additional terminal for terminating the earthing conductors.
The diagram below shows the terminals in the ceiling rose plus typical termination of the cables:
Here we have a 5 amp 4 terminal junction box that is ideal for use in lighting circuits. You will often see these used in old looped radial circuits instead of a ceiling rose in older houses. They are useful for ‘breaking in’ to an existing radial power loop to add additional wall lights in a room for example.
You can use our old friend the choc-block (nasty avoid it) but it will need to be in a suitable enclosure and kept away from anything that may give off heat (the plastic is much softer than the JB above) so you may as well use our friend above for lighting circuits.