I was explained to like this, SSD, is what you worked for and paid in... SSI is like a supplement for people who havent worked enough to give them a base line income .
Or if you qualify for SSD, and they deny you, SSI can pick you up..

I filed for SSI for fibro. 17 years ago and was turned down. My mental health provider appealed it and I got it for mental health.

SSD is most likely Social Security disability based on what you earned during your work history. SSI is a state run program that is based on low income. So if you don't qualify for SSD then you would most likely be able to get SSI. I am basing my answer on the initials.

Thanks Lisa for the information! have a good day!

@Rixthanks for your help with my question.