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In many countries, ATC services are provided throughout the majority of airspace, and its services are available to all users (private, military, and commercial). When controllers are responsible for separating some or all aircraft, such airspace is called "controlled airspace" in contrast to "uncontrolled airspace." Depending on the type of flight and the class of airspace, ATC may issue instructions that pilots are required to follow, or merely advisories to assist pilots operating in the airspace. In all cases, however, the pilot has final responsibility for the safety of the flight, and may deviate from ATC instructions in an emergency.
