Hi,
I am disabled and have service dog. If you are interested in that then you will need to have a Psych. doctor write a letter. The letter needs to state why a service dog would be benifical to you, and it does need to say "service animal"
You are correct that there are many different conditions where a service dog is helpful. You will need to discuss it over with your doctor the reasons/ and benifits it will have for you personally.
Typically a person who is fully disabled and has proof of the disabilty can have a service animal. A service animal is specificaly trained to perform certain tasks for the disabled individual. The emotional support animal, is not considered a service animal and typically can't go where ever the person goes.
The catch is that you do not need to show proof on the training a service dog has, and the dog can be trained by the disabled individual .
The burden of proof of disability does fall on the individual. Meaning a person just can't say, "hey I am disabled" w/o any docuementation.
When you asked about the severity of the disability, that is up to your doctor to decide if a service would be approiate.
In my circumstances I am on full SSDI. However, many times with a PSD medication can help a person stablize and continue to work, therfore the nature of the disability is not severe, which could indicate there is not a need for a service animal.