DCA and LGA are Level 3 airports with primarily domestic or pre-cleared international operations. Allocations are on a continuing basis based on historic slots, a two-month minimum usage requirement, and other provisions in the FAA order and rules.
Regional airports support regional economies by connecting communities to statewide and interstate markets. Local airports provide access to intrastate and interstate markets. Basic airports link communities to the national airport system and support general aviation activities.
In the U.S., the Level 2 airports include Chicago O'Hare International Airport (ORD), Los Angeles International Airport (LAX), Newark Liberty International Airport (EWR) (Change of Newark Liberty International Airport Designation, 81 FR 19861 (Oct. 20, 2016)) and San Francisco International Airport (SFO).
Category C – runways which can only be landed on by experienced pilots who have had specific simulator training. One example of a Category C airport in the UK is London City Airport. It's a relatively short runway in a very tight area, surrounded by buildings – it's got Canary Wharf at one end.