That income is considered residual income because as long as the apartment is rented and the rent is collected, the income is earned without additional effort. The effort came when the property was purchased and a tenant was found. Each month after that, the money automatically is paid without buying the apartment again or finding the same tenant each month.
Which all goes back to my point – since companies change in a lot of unpredictable ways, it makes more sense for passive income to just ride the market by investing in a Total Domestic Stock Market, Total Bond Market, and Total International index funds, with allocations that depend on your goals and time horizon. For income, withdraw 4% or less, depending on what research you believe, and you’ve got a pretty low risk strategy.
Residual income is calculated as net income less a charge for the cost of capital. The charge is known as the equity charge and is calculated as the value of equity capital multiplied by the cost of equity or the required rate of return on equity. Given the opportunity cost of equity, a company can have positive net income but negative residual income.