When a business pays a non-employee (such as an independent contractor) for services, they must obtain a Form W-9 to secure the payee ' s Taxpayer Identification Number (TIN). If the payee provides an invalid Form W-9 —meaning it is missing a TIN, has an obviously incorrect TIN, or is not signed when required—the payer is generally required by the IRS to begin backup withholding . As of current regulations, backup withholding is a flat 24% tax deducted from the gross payment and remitted to the IRS. This ensures the government receives tax revenue even if the payee fails to report the income. The " B " notice (Option D) is typically a second step triggered after the IRS notifies the payer that a TIN/Name combination does not match their records, rather than an immediate reaction to an initially invalid form.