DoDFMR 7b, Chapter 24 (
http://comptroller.defense.gov/fmr/07b/07b_24.pdf )
2403 RETIRED PAY NOT SUBJECT TO FITW
240301. The
gross retired pay of a member is not subject to FITW [Federal Income Tax Withholding] if the member’s retired pay is computed
only on the basis of percentage of disability and the member is on the temporary or permanent disability retired lists, if:
A. On or before September 24, 1975, the member was entitled to receive retired pay computed on the basis of percentage of disability (26 United States Code (U.S.C.), subparagraph 104(a)(4) and 104(b)(2)(A));
B. On September 24, 1975, he or she was a member of the Armed Forces (or Reserve Component thereof) or under a binding written commitment to become such a member (26 U.S.C. 104(a)(4) and 104(b)(2)(B)); or
C. T
he member receives disability retired pay because of a combat-related injury. The term “combat-related injury” means personal injury or sickness incurred as a direct result of armed conflict, or while engaged in extra hazardous service, or under conditions simulating war, or caused by an instrument of war.