Back in the day, the vast majority of granted Social Security disability claims were decided by Social Security's listed impairments, which set forth medical criteria of a condition that is presumed to be disabling at step 3 of the sequential evaluation. Today, however, the majority of allowed claims are determined based upon medical-vocational factors at step 5 of the sequential evaluation.
Therefore, as a practical matter, disability is functionality. Your claim will be allowed or not based upon your functional limitations, and how they affect your ability to do work-related activities. In the context of Social Security disability, this concept is expressed as Residual Functional Capacity (RFC).
Your Residual Functional Capacity is the cornerstone of your Social Security disability claim. The RFC is Social Security's assessment of your abilities to do sustained physical and mental activities on a regular and continuing basis in a work setting. The RFC considers only those funtional limitations resulting from medically determinable impairments. See 20 CFR 404.1545 and Social Security Ruling 96-8p.
Social Security looks at your ability to do basic things for an 8-hour workday, such as lifting and carrying, standing and walking, and sitting. For those with mental impairments, the SSA will assess the ability to maintain focus and concentration, to follow simple instructions, and interact with other people throughout a workday. These are very basic requirements of any employment.
The resulting RFC is used to determine whether or not you can return to your past relevant work (step 4 of the sequential evaluation) or do other work (step 5 of the sequential evaluation).
The administrative law judge must assess your RFC from the information contained in the disability claim file. Evidence of your work-related limitations can be provided by your medical records, or by medical source statements, which are opinions from your treating medical providers regarding your functional limitations. The SSA must give controlling weight to a your treating doctor's opinion, if it is "well-supported by medically acceptable clinical and laboratory diagnostic techniques and is not inconsistent with the other substantial evidence in your case record." See 20 CFR 404.1527(d)(2). This deference makes a medical source statement particularly important for diability claimants.
Non-medical evidence can also help to establish your RFC. Limits in your daily activities tend to demonstrate functional limitations. A statement from a spouse or friend may also help to establish your RFC. Testimony at the hearing regarding specific examples of your limitations can also assist the judge to assess your RFC.
With both medical and non-medical evidence, the goal fo a disability lawyer is to establish physical and/or mental limitations that preclude your ability to work on a regular and continuing basis.