The RMRS ice class rules have been revised and the class notations changed several times over the years. , the ice classes are divided to non-Arctic, Arctic and icebreaker classes. The ice class notation is followed by a number which denotes the level of ice strengthening: "Ice1" to "Ice3" for non-Arctic ships, "Arc4" to "Arc9" for Arctic ships, and "Icebreaker6" to "Icebreaker9" for icebreakers. These ice classes can be assigned in parallel with the Finnish-Swedish ice class and/or the IACS Polar Class, provided the vessel complies with all applicable rules. The selection of ice class is based on the operating area in the Russian Arctic, time of year, ice conditions, operating tactics, and whether the vessel operates under icebreaker escort or independently. In addition, icebreaker classes have additional requirements for minimum shaft power and icebreaking capability.