Residual income is different from a salary, or linear income, which is paid out strictly based on the number of hours a person works. Someone who works on a salary is often said to work “paycheck to paycheck.” This is because he pays all of his bills with his first paycheck and then must wait until he gets paid again to have more money. Ideally, someone will work hard building up a business so that he can enjoy the residual income once his goals have been met. Then he can work on additional projects while still earning money from his business.
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.

Managerial accounting defines residual income in a corporate setting as the amount of leftover operating profit after all costs of capital used to generate the revenues have been paid. It is also considered the company's net operating income or the amount of profit that exceed its required rate of return. Residual income is normally used to assess the performance of a capital investment, team, department or business unit.