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In the context of airport coordination, a slot is an authorization to either take-off or land at a particular airport on a particular day during a specified time period. This authorization is for a planned aircraft operation and is distinct from air traffic control clearance or similar authorizations. Slots, or limits on the planned aircraft operations, are a tool used in the United States and around the world to manage air traffic at extremely busy airports, and to prevent repeated delays that result from too many flights trying to take off or land at the same time.
Around the world, airports are designated at levels indicating their degree of congestion. Level 1 airports have sufficient capacity to meet demand. Level 2 airports may have some periods when demand approaches one or more capacity limits, but a voluntary schedule-facilitation process prevents systemic delays. Level 3 airports have demand for airport infrastructure that significantly exceeds the airport's capacity during the relevant periods and without controls, would have unacceptable systemic delays. Level 3 airports are under slot control and require advance approval to operate during slot controlled hours.