Registration of vehicle in category of certificate of entitlement
4.—(1)  Subject to paragraph (2), any certificate of entitlement that is issued for a motor vehicle within any category of vehicle referred to in rule 3 shall not authorise the registration of any motor vehicle within another category of vehicle.
(2)  Any certificate of entitlement that is issued for a vehicle in the open category may be used to register a motor vehicle within any other category of vehicle referred to in rule 3 (a) to (d).