For the terms military reserve, tactical reserve, or strategic reserve or "Reserve/(s)" or sometimes as a "reserve formation"...
See: Military reserve, all generally mean: military personnel or groups of units not initially committed to battle so that they are available to address unforeseen situations, bolster defenses, or exploit opportunities.
Military reserve forces, are units formed of military personnel making a long term commitment to train part-time and maintain military skills, typically sacrificing one weekend in four to serve as civilian soldiers who are available to fight when a nation mobilizes for war. Most reserve personnel have served with their regular forces before entering civilian life, and may in addition to their weekend training commitment, serve a longer annual training deployment between two weeks and a month in length.
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