I always took the "n/a" to mean self employed. While the recent LC data may say one thing, longer term unsecured consumer debt analysis told me that self employed were more likely to pay late, but defaulted less.
Couple of cases in point on 'where does the money come from', I make about 4k a month in largely passive income activities, and my girlfriend gets 80k a year in child support, both of which are considered income. Of course, her child support income stops in about 3 years, after which her income will take a huge drop if she doesn't go back to work full time.