An employee's main job location (principal place of work) is usually the place where the employee spends most of his or her working time. If there is no one place where most of the work time is spent, the main job location is the place where the work is centered, such as where the employee reports for work or is otherwise required to base his or her work.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.