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There are many different types of income. While wages, salary, and general compensation for services (called "earnings") are often the main sources of income for individuals, they aren't the only sources. There's also interest, regular dividends, rental income, distributions from pensions or retirement accounts, and Social Security benefits. Equifax collets this data with the cooperation of employers.

Equifax also relies on employers to report data to them, so if your employer doesn't, then the existence of the Work Number is irrelevant. In the era of ubiquitous side gigs where people get paid as contingency workers rather than wages or salaries by the company, you run into issues relying on the Work Number, even before factoring in all the other types of income.