According to the Defense Travel Reg, "Unused passenger space on DoD-owned and controlled passenger-carrying aircraft, on all types of missions (e.g., channel, SAAM, Operational Support Airlift (OSA), Aeromedical Evacuation) be utilized to the maximum extent possible, to include Space Available (Space-A) passengers, as authorized by DODI 4515.13. Available seats will be released for Space-A travel unless overriding safety or legal concerns (including hazardous cargo, CBP, agriculture, or immigration considerations), or a defined need for security prohibit Space-A travelers from flying on a specific mission." (DTR Part I, Passenger Movement, Chapter 103).
The DoD Policy is reiterated in AMCI 24-101 V14, Para 4.9.2., "IAW DoD policy, all unused seats on DoD-owned or controlled airlift will be released for use by Space-R and SpaceA passengers. On all mission types, the senior AMC or terminal representative, in coordination with the aircraft commander and the user as applicable, will determine if a defined need for security, or if overriding safety or legal concerns (including hazardous cargo, customs, agriculture or citizenship and immigration service consideration) prohibits Space-A travelers from flying on a specific mission. The aircraft commander is the final authority responsible for all matters affecting the operation of their aircraft. This authority is to be exercised judiciously within the context of DoD Space-A policy stated in Para 10.2. Situations where the above prohibitions restrict Space-A travelers from a particular mission should be the rare exception rather than the rule."