That is sent to you for you to declare any Miscellaneous Income you receive that is not part of your employment. The obvious thing is bonuses paid as a lump sum in cash, or if you do work for another company are weekends and are paid in Cash. Send it back marked 'Not Applicable' or 'Nil' in the boxes. They will let you know if they have 'information' that you have another income.
Payments to others
If you are involved in a trade or business and receive payments from other people or entities, these transactions will likely be reported to you and the IRS if they total more than certain amounts during the tax year. Depending on the nature of these payments, the IRS requires the amounts paid to be reported on either Form 1099-MISC in the case of payments for rent, royalties, prizes and awards, substitute payments in lieu of dividends and other items or 1099-NEC if the payments represent nonemployee compensation.
Prior to tax year 2020 these payments usually appeared on Form 1099-MISC. Now, businesses must separately report nonemployee compensation on
Form 1099-NEC. Any payments made to employees, including income, retirement contributions, insurance payments, expense reimbursements and travel expenses, typically go on
Form W-2. Other than the change for nonemployee compensation, the use of Form 1099-MISC remains mostly the same for payment reporting.
https://www.google.com/search?q=1099+MISC
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