Which document serves as an employee's withholding allowance certificate?

The withholding allowance certificate for an employee is specifically represented by Form W-4. This document is crucial as it enables employees to inform their employer of the amount of federal income tax to withhold from their paychecks. By providing information about their filing status, number of dependents, and any additional income or deductions they may be eligible for, employees can effectively manage their tax withholdings throughout the year. This helps ensure that they either meet their tax obligations accurately or potentially receive a refund when they file their annual tax return.

In contrast, other forms like Form W-2 serve as a summary of an employee's annual earnings and the taxes withheld by the employer, while Form 1040 is the individual income tax return form used to report income and calculate taxes owed. Form 1099 is used by independent contractors and others to report various types of income received outside of traditional employment, rather than as a tool for determining withholding allowances. Thus, Form W-4 is the sole document specifically designed for managing tax withholdings for employees.