When the SSA receives your initial disability claim, it reviews your medical records, and determines your RFC. The problem with this approach is that the SSA does not obtain your doctor's opinion about your limitations. Rather, the SSA reviews your medical records and makes its own determination.
Social security never sends an RFC form to an applicant's own doctor. Instead, they have one of their own doctors—referred to as a medical consultant and assigned to a case processing unit with disability examiners—complete the RFC.
The problem, of course, is that a doctor's treatment notes are intended to manage a patient's medical care, not to establish the functional limitations that Social Security uses to evaluate a claimant's ability to work. As a result, the method used by Social Security to evaluate disability often results in a denial of the initial claim.
To remedy this situation, it is very important to obtain your doctor's opinion regarding your physical and/or mental limitations, and how those limitations affect your ability to work. A medical source statement from your doctor setting forth your limitations due to your impairments is usually the most powerful evidence in support of your disability claim.
For more information on this topic, read Tomasz Stasiuk's excellent post The One Element Missing in Most Social Security Cases: A Medical Opinion of Limitations on his Colorado Social Security Disability Law blog.