What is the purpose of Form W-9?

Form W-9 is Request for Taxpayer Identification Number and Certification. Your taxpayer identification number (TIN) is your social security number. If you're a resident alien and you're not eligible for a social security number, then use the individual taxpayer identification number (ITIN) assigned to you by the IRS. If you're a sole proprietor and you have an Employer Identification Number (EIN), then you can use either your social security number or your EIN.

The purpose of Form W-9 is to provide your correct TIN to the requester (the person requesting it). The requester is required to file an information return with the IRS to report, for example, mortgage interest you paid, cancellation of debt, contributions to your IRA, real estate transactions, income paid to you, etc. In order to report this information to the IRS, the requester needs your correct TIN.