Cat D is 'uneconomical to repair' which is when it's over 60% of the car's value (or thereabouts). Often issued against stolen recovered cars. The car doesn't need a VIC check to go back on the road.

Cat C is where the cost of repair is greater than the value of the vehicle (using main dealer new parts). As with Cat D the car is reparable, but it can't be used on the road until a VIC check has been passed.