The strict version most sellers use: a note is non-performing when the borrower has stopped paying and the loan is in default, usually meaning 90 days or more past due. A performing note is current, or close enough that payments keep arriving. Sub-performing is the loose middle, and it's where the sloppiness you spotted lives. Some sellers use it for a borrower who pays late but pays, some use it for a loan that hasn't paid in a year but where the borrower is still talking to the servicer. There's no rule forcing them to be consistent, so on any tape you have to look at the actual pay history rather than the label.
The number to ask for is the last paid date, not the days delinquent, because days delinquent can be reset by odd accounting. Ask for a full pay history and a payoff statement showing principal, accrued interest, escrow advances, and any fees.
One thing that catches people at your stage: buying the note means buying the debt, not the house. If the borrower never resumes paying, your route to the house runs through foreclosure, and foreclosure timelines and procedures vary a great deal by state, from a few months in some to a couple of years in others. That timeline is a cost, because you're carrying taxes and possibly insurance while you wait. So the same loan balance is worth different money in different states.
