Investment income relative to wealth is the best classifier for wealth class. When one person's net annual income is another's net wealth, they're in different classes. This puts most people in the .middle class with about 100k income through work or investment. That bracket ends at about 2m net worth as they make about 100k off their investments. Then you hit 40m where the 40m net worth person is making 2m a year. Then 800m at 40m per year. Then 16bn at 800m per year. At the end of the day, everyone past tier 1 doesn't have to work, but there is a lot going on between each of those layers.