The following is from the downloadable document at the bottom of the page that provides a list of those swimmers who are eligible for competing in disability swimming:

Classification Eligibility Guidelines for Disability Swimming(S1-S15)

Classification groups S1-S15 are for those swimmers with a functional, visual, learning or hearing impairment.

S1 - S10 swimmers with a functional (physical) disability.

The following disabilities meet the eligibility requirements of the functional classification system*:
Achondroplasia (dwarfism)
Amputees (including dysmelia)
Arthrogryposis
Brittle bones
Cerebral Palsy
Legge-Perthes
Multiple Sclerosis
Muscular Dystrophy
Polio
Spina Bifida
Spinal Cord injury

* Please note that this list is not exhaustive, however provides a guide to functional classification eligibility

S11 - S13 swimmers with a visual impairment
The range here is from blind to visually impaired (best eye with best correction)

S14 swimmers with a Learning Disability
Swimmers must have an IQ of 75 or less to meet the criteria

S15 swimmers with a hearing impairment
Any swimmer who wears a hearing aid in both ears will be considered

There are a number of conditions that do not meet the eligibility criteria as follows*:
ADHD
Aspergers
Asthma
Autistic spectrum
Cystic Fibrosis
Diabetes
Dyspraxia
Epilepsy
Haemophilia
Hydrocephalus
Obesity
Learning Difficulties
Scoliosis
Transplants

* Please note that this list is not exhaustive, however provides a guide

It must be noted that even those swimmers that have a condition eligible to be classified may still not be sufficiently impaired to gain a classification.