The Social Security Disability system is a flexible one that is not intended to jam everybody into the same limited box. While there is naturally some degree of rigidity built into any government regulation,the idea behind the system is to actually help people in their own individual set of circumstances rather than force them to strain in order to meet an inapplicable group of requirements.
The primary focus of Social Security disability criteria is to determine what sort of job you have engaged in up to this point in your life,whether or not you are still able to perform that job,and if there is any possibility of retraining you to perform some sort of other job that your disability does not interfere with the performance of ( SSD ).
If you have spent the bulk of your working career as a lumberjack and now have a bad back,it is most unlikely that you will ever be able to go back to that job again. Yet it is equally unlikely that you can become an astronaut or immunologist as a replacement career. There may,however,be something which you can be retrained to do and the system wants to help you find out what it is and what can be done to get you there.
This is the point of establishing a level of disability as part of the process. If you are paralyzed from the neck down,there is effectively nothing that you can do ever again and you are totally and permanently disabled. Others may be just partially disabled and capable of doing something else. In some cases,this may represent a significant fall in income between the old and new types of job one can hold. The purpose of the system of disability criteria is simply to establish what is the most effective way to help you at the least possible burden to society.