Note: You must complete a separate Form 3911 for each refund for which you are requesting information. Use Form 843 to claim a refund or request an abatement of certain taxes, interest, penalties, fees, and additions to tax.Check the box below that indicates your reason for filing Form 843. Tax. Abatement or refund of tax other than income, estate, or gift tax. Use Form 843 to claim a refund or request an abatement of certain taxes, penalties, additions to tax, interest, and fees. The primary letter we use to request information about a return is Form DTF948 or DTF948O, Request for Information (RFI). The Illinois Department of Revenue (IDOR) sends letters and notices to request additional information and support for information you report on your tax return. We're examining your tax return. What you need to do. Each nontax filer is required to provide the letter.