There are lines somewhere near I’ve drawn as shown above. That’s roughly how income groups are divided. If you click the ‘Original Income’ below the income screen, you will see original distribution of people’s income. These incomes will be modified by various factors such as Income Tax (-), Private Heathcare (-), Bus Subsidies (+), State Pensions (+), or Wages (+/-). Modified income is called disposable income here and it ultimately decide who belongs to which income group. Let’s see an example here.
Person A is Capitalist, Liberal, Environmentalist, Commuter, and State Employees. He has original income of $ XXXXX and it will be in the range of middle income. Now he pays Income tax at middle-income rate, gets some tax credit (which doesn’t benefit the poor), spends money on essential parts like healthcare & education (unless the gov covers them all or private ones are banned), happens to realize he have more money than before thanks to high wage level, and many more. He also have other income modifiers such as pro-environment policies benefiting environmentalists, income boosts for State Employees from public spending programs, and possibly Bus Subsidies making it cheaper to commute to work. Now his income will be very different from the original one. He may become slightly poorer if taxes & expenses were too high and even be re-classified as poor.
This happens to every single dot in the disposable income screen.