Space-A travel is a means by which members of United States Uniformed Services (United States Military, reservists and retirees, United States Department of Defense civilian personnel under certain circumstances), and these groups' family members, are permitted to travel on aircraft under the jurisdiction of the United States Department of Defense when excess capability allows.
Space available travel is a privilege that derives, in part, from United States Code, title 10, section 4744, which states, "officers and members of the Military Departments, and their families, when space is available, may be transported on vessels operated by any military transport agency of the Department of Defense". Space available travel is defined as "travel aboard DoD owned or controlled aircraft and occurs when aircraft are not fully booked with passengers traveling under orders".
It is a privilege offered to United States Uniformed Services members. Retired members are given the privilege in recognition of their career and because they are eligible for recall to active duty. The criteria for extending the privilege to other categories of passengers is their support to the mission being performed by Uniformed Services members and to the enhancement of active duty Service members' quality of life.
There are rules and guidelines which apply to such travel. Uniformed personnel may only travel Space-A while on leave or pass for the full duration of their Space-A trip, and Space-A travel cannot be used in conjunction with travel required by the service. Space A travel may not be used for personal financial gain or in connection with business enterprises or employment. Other nations' laws and policies, as well as U.S. foreign policy, may limit the ability to travel using Space-A.
Aside from members of the United States Marine Corps, travelers do not have to be in uniform for their flights.
Eligible passengers wanting to travel using DoD Space-A travel are required to sign up at the departing location and are then placed on a locally managed Space-A register. The registration process varies depending on the location, but most locations allow signups via electronic mail, fax, or postal mail.