Illinois Form W9-Request for Taxpayer Identification Number and Certification is an official form issued by the Illinois Department of Revenue. This form is used by businesses and individuals to obtain their own taxpayer identification number (TIN). It is also used to certify that the information provided on the form is accurate and complete. There are two types of Illinois Form W9-Request for Taxpayer Identification Number and Certification: an individual form and a business form. The individual form is used by individuals to obtain their own taxpayer identification number and certify that the information provided is accurate and complete. The form requires the individual to provide information such as name, address, Social Security number, and other identifying information. The business form is used by companies and organizations to obtain their own taxpayer identification number and certify that the information provided is accurate and complete. The form requires the company to provide information such as name, address, employer identification number, and other identifying information. Both forms must be signed and dated by the taxpayer before they can be used.