Under arms describes a state of military readiness (actual or ceremonial). Typically, troops are considered "under arms" when they are in uniform, on duty, and carrying a weapon (rifle, side-arm, or sword), as opposed to being in uniform, on duty, but not carrying a weapon.
Soldiers not actually carrying a weapon but wearing a service belt or web gear associated with carrying weapons may be considered to be symbolically "under arms".
A regular army is the official army of a state or country (the official armed forces), contrasting with irregular forces, such as volunteer irregular militias, private armies, mercenaries, etc. A regular army usually has the following:
A military reserve force is a military organization whose members (reservists) have military and civilian occupations. They are not normally kept under arms, and their main role is to be available when their military requires additional manpower. Reserve forces are generally considered part of a permanent standing body of armed forces, and allow a nation to reduce its peacetime military expenditures and maintain a force prepared for war. During peacetime, reservists typically serve part-time alongside a civilian job, although most reserve forces have a significant permanent full-time component as well. Reservists may be deployed for weeks or months-long missions during peacetime to support specific operations. During wartime, reservists may be kept in service for months or years at a time, although typically not for as long as active duty soldiers.
In countries with a volunteer military, reserve forces maintain military skills by training periodically (typically one weekend per month). They may do so as individuals or as members of standing reserve regiments—for example, the UK's Army Reserve. A militia, home guard, state guard or state military may constitute part of a military reserve force, such as the United States National Guard and the Norwegian, Swedish and Danish Home Guard. In some countries (including Colombia, Israel, Norway, Singapore, South Korea, Sweden, and Taiwan), reserve service is compulsory for a number of years after completing national service. In countries with conscription, such as Switzerland and Finland, reserve forces are citizens who have completed active duty military service but have not reached the upper age limit established by law. These citizens are subject to mandatory mobilization in wartime and short-term military training in peacetime.