41 C.F.R. § 101-26.102-2
Utilization by military agencies
Military activities shall utilize the buying services of GSA when:
(a) GSA has agreed with the Secretary of Defense, or with the Secretary of a military department in connection with the requirements of that department, to perform such buying services; and
(b) The items involved are not properly obtainable from GSA stock or Federal Supply Schedules.