Under most group disability policies you must be able to show with a relative degree of certainty that you meet the contractual definition of disability. Many well-meaning physicians will tell a patient that "it is clear you are disabled" and then they will write a letter that says "it is clear my patient is disabled."
This isn't good enough.
The file that you submit to a disability insurance company should contain clear objective evidence which supports your claim to disability. Many times your doctor may be exactly right however his opinion needs to be expressed in a certain way and typically backed up with certain objective medical testing and records.
Each case is different but essentially you're going to be teaching the claims adjuster that your claim is valid. Ultimately, if your claim is denied or terminated early you will have the right to appeal that decision back to the insurance company. If the claim is still denied you will typically have the right to bring a lawsuit in federal court.
The only thing the judge is going to be looking at is the record that you have developed with the insurance company. The closer he letters from doctors that say "my patient is disabled" almost never win the day for the claimant.
An experienced long-term disability attorney can help you put together a winning claim or appeal. It is usually a bad idea to appeal a disability claim denial on your own or to even let your doctor draft the appeal by sending a letter. Your proof of disability must be consistent with the policy terms and with established law and cases in your jurisdiction.