Backup withholding warning notification. When a 1099 is missing the payee taxpayer identification number (TIN) or the TIN is incorrect, the IRS sends a notification (form CP2100 or CP2100A) to the payer. The B-notice is the notice that the payer sends to the payee, though sometimes the CP2100 is referred to as a b-notice. The B-notice alerts the payer that within 15 days the payer must contact the payee and request a W-9 with a correct TIN or it will begin backup withholding on further payments. The purpose of B-Notices is to ensure that accurate TIN information is provided by payees to payers for tax reporting purposes. It’s important for payees to respond to B-Notices and provide correct TIN information to avoid backup withholding and potential penalties. See IRS Publication 1281.