There are generally 2 types of notes, performing, and nonperforming (there are also reperfroming, which are formerly nonperformings, performing currently).
There are notes for real estate, both the single family home, and multifamily (eg, apartments), as well as commercial real estate (hotels, stores, big box stores, etc). There are also notes for businesses.
Generally, note investors like real estate notes, because there is something solid there, that they can easily resell, if they must foreclose. Business notes, not so much. It can be hard to value equipment, cash flow, invoices, and other factors, whereas the real estate can be easy, with only the one asset.
Here is one place to look at, regarding performing notes.