It can be used on its own although you would get the best performance with the matching rear pulley. The clutch would be extra.

These can not be used with an electric start, the kit includes a spacer to replace the starter components. 

The Stage6 oversize variator kit can be used on it's own with a standard belt and it works very well.

It holds the revs more steadily than the regular Sport Pro Stage6 variator (which already works very well) but be aware it is happiest revving high (something like a high revving sport or mid-race cylinder kit upwards), although probably anything 10,000rpm upwards would be fine.

If you fit the complete kit with the different rear end and belt it has the further advantage of a stronger thicker wider belt, with lighter stronger torque assembly optimised for and capable of withstanding high end hi-revving race cylinders etc..
Because it is 'oversize' you also get the advantage that it can hold the revs steady over a wider speed range (=higher top speed) because the belt has a larger range of movement up and down the cones.
So say for example you have a gearup kit, a standard variator might be able hold the revs steady up to say 50 mph (actual speed depends on many factors!) after which the belt is moved all the way out and the only way the bike can then go faster is if the revs increase (effectively becomes a fixed gear bike like a geared motorbike). An oversize kit (as opposed to just the variator) would allow it to hold the revs steady up to say 54 or 55mph in the previous example.