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Active duty refers to a full-time occupation as part of a military force, as opposed to reserve duty. In Great Britain and the Commonwealth the equivalent term is active service.
The Indian Armed Forces are considered to be one of the largest active service forces in the world with almost 1.42 million Active Standing Army. An additional 2.20 million reserve forces can be activated in a few weeks as per the situation under the order of the President of India who is the Commander and Chief of the Armed Forces of India. This does not include the additional 1.40 million troops of the Paramilitary who too are an active force whose full-time responsibility is to guard the sovereignty of the nation from internal and external threats.
In the Israel Defense Forces, there are two types of active duty: regular service (Hebrew: שרות סדיר, Sherut Sadir), and active reserve duty (Hebrew: שרות מילואים פעיל Sherut Milu'im Pa'il, abbr. Shamap). Regular service refers to either mandatory service (Hebrew: שרות חובה, Sherut Hova), according to the laws of Israel, or standing army service (Hebrew: שרות קבע, Sherut Keva), which consists of paid NCOs and officers.
Active reserve service refers to the actual time in which reservists are called up. This varies from once every few years to a month every year. During active reserve duty, military law can be applied to reservists, similarly to regular soldiers.
The Pakistan Armed Forces are one of the largest active service forces in the world, with almost 617,000 full-time personnel, due to the complex and volatile nature of Pakistan's relationship with India and the Kashmir region, and its porous border with Afghanistan. An additional 513,000 part-time reservists (including armed civilians of FATA) can be activated in a few weeks as per the situation under the order of the President of Pakistan who is the Commander and Chief of the Armed Forces of Pakistan. This does not include the additional 427,627 troops of the Paramilitary who too are an active force whose full-time responsibility is to guard the sovereignty of the nation from internal and external threats.
United States 
In the United States military, active duty refers to military members who are currently serving full-time in their military capacity. Members of a reserve component are not generally considered active duty. However, in support of the Global War on Terror and other contingency operations, a large number of Reservists in all branches have been called to active duty in an operational capacity. Many will argue that today's Reserve forces are no longer the "strategic" Reserve of the Cold War. Those Reservists deployed in support of contingency operations, either as a unit or by individual augmentation, are also considered active duty. These terms may also be applied to military forces of other nations, although the details concerning obligations to serve may differ.