According to DOD Instruction 4500.57, Transportation and Traffic Management, Section 4.3:
Unused passenger space on DoD-owned and -contracted passenger-carrying aircraft, on
all types of missions (e.g., channel, special assignment airlift mission, operational support airlift,
aeromedical evacuation) will be used to the maximum extent possible, including space-available
passengers, pursuant to DoDI 4515.13.
Available seats will be released for space-available travel unless overriding safety (e.g.,
training flights) or operational concerns (e.g., hazardous cargo, customs and border protection,
agriculture, or immigration considerations) or a defined need for security prohibit space-
available travelers on a specific mission.
Situations where the above prohibitions restrict Space-A travelers from a particular mission should be the rare exception rather than the rule."