Bunker fuel or bunker crude is technically any type of fuel oil used aboard vessels. Its name is derived from coal bunkers, where the fuel was originally stored. The Australian Customs and the Australian Tax Office defines a bunker fuel as the fuel that powers the engine of a ship or aircraft. Bunker A is No. 4 fuel oil, bunker B is No. 5, and bunker C is No. 6. Since No. 6 is the most common, "bunker fuel" is often used as a synonym for No. 6. No. 5 fuel oil is also called Navy Special Fuel Oil (NSFO) or just navy special; No. 5 or 6 are also commonly called heavy fuel oil (HFO) or furnace fuel oil (FFO); the high viscosity requires heating, usually by a recirculated low pressure steam system, before the oil can be pumped from a bunker tank. Bunkers are rarely labeled this way in modern maritime practice.