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".
Retired members are given the privilege in recognition of their career and because they are eligible for recall to active duty.
Space A travel may not be used for personal financial gain or in connection with business enterprises or employment.
Unlike traditional commercial air traffic, military flights are not always assigned predictable takeoff times.
DoD reg, Section 4, Table 3, a Gray Area Retiree (Reservist who is "eligible" for retirement pay at 60 years of age but not fully retired with a BLUE ID Card), can fly Space-A only within the CONUS and directly between the CONUS and Alaska, Hawaii, Puerto Rico, the US Virgin Islands, Guam, and American Samoa (Guam and American Samoa travelers may transit Hawaii or Alaska); or traveling within Alaska, Hawaii, Puerto Rico or the Virgin Islands.
As of August 13th 2018, authorized veterans with a permanent service-connected disability rated 100% as total are eligible for Space-A travel traveling in the CONUS or directly between the Continental United States CONUS and Alaska, Hawaii, Puerto Rico, the U.S. Virgin Islands, Guam, and American Samoa (Guam and American Samoa travelers may transit Hawaii or Alaska); or traveling within Alaska, Hawaii, Puerto Rico or the U.S. Virgin Islands.
The following dependents and family members are NOT eligible to fly Space-A at all: The majority of flights that passengers take occur on: C-5, C-17, C-40, C-130, KC-10, and KC-135 aircraft.