sentences:  More watertight bulkheads, in addition to those required by a ship's normal class, are usually required.In addition, heating arrangements for fuel tanks, ballast tanks, and other tanks vital to the ship's operation may also be required depending on the class.Ships can be assigned one of seven Polar Classes (PC) ranging from PC 1 for year-round operation in all polar waters to PC 7 for summer and autumn operation in thin first-year ice based on the "Unified Requirements for Polar Class Ships" developed by the International Association of Classification Societies (IACS).