Swimmers with a disability (para swimmers) have classification based on their level of impairment.
S1-10 is physical impairment. S1 being most severe (usually no arms or legs) to S10. S10 swimmers usually miss a hand, have restrictions in 1 hip joint.
S11-13 are visually impaired.
S14 is intellectual impairment.
You apply for classification through British swimming and once you've completed a mound of paperwork and waited in a long queue you are assessed. For physical disability You score out of 5 for movements (tests depend on your type of disability). Each set of tests give a max of 300 points. If you lose more than 15 during movement tests you are tested in water.
If when ended your score less than 285 you have a classification.
An S10 is 266-285. S 9 246-265 (iirc!)
Visual and intellectual impairments are tested differently (for obvious reasons!)
Swim england (and I assume wales and Scotland) will have their own talent programmes and structure for these. Some counties will have their own as well.
They invite swimmers from different age groups to be part of a programme based on their times and rankings and sometimes performance if they show potential. Some will get invited to the next phase. Some will get invited the following year. Some may be invited one year and then progress slow but pick and and be invited again.
By the older age groups it's the swimmers you see winning multiple medals at nationals and usually in more than 1 stroke.