What is the meaning of space available mail?

(DoD) A transportation category for military mail transported to and from overseas bases by air on a space-available basis. Also called SAM.
Department of Defense Dictionary of Military and Associated Terms. 31 January 2011.
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APAMilitary Dictionary (DoD). (2011). space available mail. Retrieved September 22, 2026, from http://smartdefine.org/space_available_mail/definitions/1964023
ChicagoMilitary Dictionary (DoD). 2011. "space available mail" http://smartdefine.org/space_available_mail/definitions/1964023 (accessed September 22, 2026).
HarvardMilitary Dictionary (DoD) 2011, space available mail, Smart Define, viewed 22 September, 2026, <http://smartdefine.org/space_available_mail/definitions/1964023>.
MLAMilitary Dictionary (DoD). "space available mail" 4 April 2011. Web. 22 September 2026. <http://smartdefine.org/space_available_mail/definitions/1964023>