SSDI is Social Security. SSI is not. SSI is supplemental help to disabled who do not meet the number of hours of work needed to qualify for Social Security. Sometimes though, the amount of Social Security one might qualify for, if one qualifies, is not as much as one might need to live on in your area (Social Security and how much one gets is standardized) so you also get the supplemental amount which can be standardized (it's passed out to states by the Feds) or your state might add to it for its citizens, etc. so the combination would be more than you'd get with just SSI. However, the tax status and extra benefits one is entitled to (Medicare versus Medicaid) might differ also. Apparently Minnesota has MSA?

DB101 Minnesota - Minnesota Supplemental Aid (MSA): The Basics

Are you sure you were talking to a Social Security person? If so and you don't understand, maybe calling or going in to see your State benefits person/department, maybe they can explain it better.