The Form W-9 question on Form 1040-C is related to the requirement for departing aliens to provide their taxpayer identification number (TIN) to any person who is required to withhold income tax on their payments to the alien. This information is used to ensure that the correct amount of tax is withheld and reported to the IRS. Departing aliens should provide their TIN as soon as possible to avoid delays in the processing of their tax return.