It is a function-based classification system designed to allow for fair competition in disability swimming. The classes are prefixed with "S" for freestyle, butterfly and backstroke events, "SB" for breaststroke and "SM" for individual medley events. Swimmers with physical disabilities are divided into ten classes based on their degree of functional disability. Those with visual impairments are placed in three additional classes. A final class, S14, is reserved for swimmers with intellectual disabilities, also know as Special Olympics.