nationals- how do u define each category?

just wondering, maybe dave can asnwer this. how do i know if a certain car qualifies for a particular category?
could i enter the unfinished project coz there is more to do on it?
wat makes a car stock? wat happens if the car was dechromed only, is that 'modified'? wat about just wider tyres?
i wont enter anything but just for interests sake.
thanks
sam