The first time I heard this phrase was in Stephen Ambrose's book of the same name. (LINK) I think the term as used by Ambrose is meant to distinguish temporary combatants from "professional soldiers," men (and now women) who have joined the military as a career. The latter would include NCOs and commissioned officers, and especially graduates of military academies, although some of those do leave the service promptly upon fulfilling their required service.

The cadre of professional soldiers is often referred as "Regular Army." For example, on the television program M*A*S*H, Col. Potter was Regular Army as he had been a solider as far back as WWI when he was in the cavalry. Hawkeye and Trapper were most definitely NOT Regular Army. Citizen soldiers may be either draftees or volunteers, and it is understood that when hostilities are over they will return to civilian life and resume other careers.