What is a "casual" interview?

I recently started looking for work after being home with my family for 13 years. I'm a little out of the loop on how things works these days. I have an interview on Monday, and the contact specifically called it a casual interview. Is this a thing, and if so, what makes it different from a regular interview? Should I dress less formally?

Also, the contact instructed me to bring ID and SS card. I'm not really comfortable with providing my SS card before an offer, which was pretty standard in the past. Has this changed? Would it be bad form to decline providing it during the interview?

Thanks so much!

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