What is the form designated for back-up withholding tax?

The form designated for backup withholding tax is Form 945. This form is specifically used by payers to report federal income tax withheld on certain payments, including payments made to contractors or for other non-employee compensation, when backup withholding is required. Backup withholding usually applies when the taxpayer's identification number (such as a Social Security number or Employer Identification Number) is not provided, is incorrect, or under certain reporting requirements.

Other forms mentioned do not serve the purpose of reporting backup withholding. For example, Form 940 is used to report unemployment tax, while Forms 941 and 944 are used for reporting federal income taxes withheld and employers' and employees' contributions to Social Security and Medicare. Lastly, Form W-2 is the wage and tax statement provided to employees that summarizes their annual earnings and withholding amounts but does not specifically address backup withholding.