The following information is taken from A Swimmer's Guide to Functional Classification.

The functional classification system (FCS) is a series of tests and observations designed to determine each swimmer's capability to move through the water. This system is used for all athletes who have physical disabilities. Each swimmer can have up to three classifications: an "S" class for freestyle, backstroke, and butterfly; an "SB" class for breaststroke; and an "SM" class for the individual medley. There are ten "S" classes, nine "SB" classes, and ten "SM" classes. The following profiles are based upon the December 1998 edition of the International Paralympic Committee Swimming Classification Manual, and was taken from a summary developed by Gail M. Dummer, posted on USA Swimming Adapted Swimming: USA Swimming Organization Search information

Class	Profile
S1	Unable to catch the water. Severely limited propulsion from the arms due to muscle weakness, restricted range of motion, or uncoordinated movements.
S2	Unable to catch the water or control wrist. Limited propulsion from the arms due to muscle weakness, restricted range of motion, or uncoordinated movements.
S3	Inability to catch the water effectively with limited wrist control. Limited propulsion from the arms due to muscle weakness, restricted range of motion, or uncoordinated movements.
S4	Wrist control and some finger control. Arm movements are propulsive, but may not be well coordinated. Minimal to no trunk control.
S5	Control of the wrist and fingers. Arm movements are propulsive, but may not be well coordinated. Limited trunk control. Legs provide balance or minimal propulsion.
S6	Able to catch the water. Efficient arm movements with satisfactory propulsion. Good control of upper trunk. Limited to good leg propulsion. May use dive, assisted dive, or water start.
S7	Controlled catch phase. Efficient, propulsive arm movements, except in cerebral palsy where function decreases as stroke rate increases. Trunk control varies, but usually some limitations. Limited to good leg propulsion. May use dive, assisted dive, or water start.
S8	Effective catch. Efficient, controlled, propulsive arm movements. Minimal loss of trunk control. Limited to good leg propulsion. May use dive, assisted dive, or water start.
S9	Effective catch. Efficient, controlled, propulsive arm movements. Full trunk control. Propulsive kick. Dive start.
S10	Effective catch. Efficient, controlled, propulsive arm movements. Full trunk control. Strong kick. Dive start.
FCS:	Profiles of the "SB" classifications
SB1	Unable to catch the water. Severely limited propulsion from the arms due to muscle weakness, restricted range of motion, or uncoordinated movements.
SB2	Weakness holding the hand in a flexed position and inability to catch the water effectively. Limited power in arm movements due to muscle weakness, restricted range of motion, or uncoordinated movements.
SB3	Wrist control and some finger control. Limited propulsion from the arms due to muscle weakness, restricted range of motion, uncoordinated movements, or short limb length.
SB4	Most swimmers have good control of the wrist and fingers in the catch phase. Arm movements are propulsive, but may not be well coordinated or powerful. Limited trunk control.
SB5	Able to catch the water. Satisfactory arm cycle with power during propulsive phase. Satisfactory trunk control. Most swimmers in this class do not have a functional kick and have some leg drag, though some show intent to kick, propulsion with one leg, or (dwarfs) a functional kick.
SB6	Most swimmers in this class are able to catch the water. Most have full control of arm movements and satisfactory propulsion. Good control of upper trunk. Leg movements vary from none to intention to kick to propulsive kicking movements depending upon disability profile.
SB7	Controlled catch phase. Full controlled arm cycle for most swimmers. Minimal or no loss of trunk control. Leg movements vary from intention to kick to propulsive kicking movements depending upon disability profile.
SB8	Effective catch. Full controlled arm cycle. Minimal or no loss of trunk control. Limited to good leg propulsion.
SB9	Effective catch gaining full power. Full controlled arm cycle. Full trunk control. Propulsive kick with some involvement in affected limbs.
FCS:	Profiles of the "SM" classifications There are three classifications for swimmers who are blind/visually impaired.
S11	Swimmers who are totally blind. May possess light perception, but are unable to recognize hand shapes at any distance.
S12	Swimmers with visual acuity up to and including 20/600, or a field of vision limited to less than 5 degrees.
S13	Swimmers with visual acuity greater than 20/600 but less than 20/200, or a field of vision ranging from 5 to 20 degrees.
S14	Cognitive disabilities, may include mental retardation, severe learning disability, or autism. Eligibile swimmers have an IQ score of 70 points or less. Most swimmers who qualify for Special Olympics competition are eligible to compete in the S14 category.
S15	Includes swimmers who are deaf or hard of hearing.