41 C.F.R. § 302-17.5
The withholding tax allowance (WTA) and the relocation income tax allowance (RITA) are the two allowances through which the Government reimburses you for substantially all of the income taxes that you incur as a result of your relocation. You are eligible for the WTA and the RITA if you are relocating in the interest of the Government, and your agency's reimbursements to you for relocation expenses result in you being liable for additional income taxes. Eligibility for WTA and RITA includes, among others, transferred employees, appointments (new or political), assignments under the Government Employees Training Act, and those returning from an overseas assignment for the purpose of separation from Government service.
41 C.F.R. §302-17.5
Note to § 302-17.5: If your agency offers you the choice, the WTA is optional to you. See 302-17.61 through 302-17.69.