A 1099 document in mortgage processing refers to IRS Form 1099, which is used to report various types of income, including income from interest, dividends, and other financial transactions. In the context of mortgage processing, Form 1099 may be used to report income from investments, rental properties, or other sources that a mortgage applicant has. Lenders often require mortgage applicants to provide these forms as part of their financial documentation to assess the applicant's income and financial stability.