What doccumentation does one need to claim disabled for tax purposes? This person has applied for SSI, but a determination has not been made yet.

[xmasbaby0]

You have not mentioned whether the person you are asking about is a child or an adult.

Do I get a credit or deduction if my child is disabled?

If your child is older than 12 and you need to pay someone to take care of him while you work, you can say he is disabled and you can still get the childcare credit on your federal return.   Or if he is 19 or older you may still be able to claim him as a qualifying child dependent on your federal return.

Aside from those things, there is no federal tax advantage to saying that your child is disabled.   Your state tax laws might be different.

For disabled adults:

There is an extra amount added to your standard deduction if you are legally blind.   Your federal return does not provide any other credits or deductions for saying you are disabled, with one possible exception.  

There is a credit for the Elderly and Disabled which is so small that very few people benefit from it—-it has not been updated/increased by Congress for decades.

ELDERLY OR DISABLED CREDIT https://www.irs.gov/help/ita/do-i-qualify-for-the-credit-for-the-elderly-or-disabled

There are some states that provide various credits to elderly/disabled folks, so watch for that when you prepare your state return.   The states that I know of that have anything for elderly/disabled are AZ, CT, DE, IA, MO, ND, NJ, NY, VE.   There may be others.

[Vanessa A]

You generally don't need to send documentation in to the IRS to show someone is disabled with your return.

  However, it is a good idea to keep a statement from the Dr. saying they are disabled if the disability is the reason you are able to claim them or a credit for them.