IFR, SVFR, or VFR. All flights are subject to ATC clearance (country-specific variations notwithstanding). Aircraft operating under IFR and SVFR are separated from each other, and are given traffic information in respect of VFR flights. Flights operating under VFR are given traffic information in respect of all other flights. Class E: Operations may be conducted under IFR, SVFR, or VFR. Aircraft operating under IFR and SVFR are separated from each other, and are subject to ATC clearance. Flights under VFR are not subject to ATC clearance. As far as is practical, traffic information is given to all flights in respect of VFR flights. Class F: Operations may be conducted under IFR or VFR. ATC separation will be provided, so far as practical, to aircraft operating under IFR. Traffic Information may be given as far as is practical in respect of other flights. Class G: Operations may be conducted under IFR or VFR. ATC has no authority but VFR minimums are to be known by pilots. Traffic Information may be given as far as is practical in respect of other flights. Special Airspace: these may limit pilot operation in certain areas. These consist of Prohibited areas, Restricted areas, Warning Areas, MOAs (military operation areas), Alert areas and Controlled firing areas (CFAs), all of which can be found on the flight charts. Classes A–E are referred to as controlled airspace. Classes F and G are uncontrolled airspace. The table below provides an overview of the above classes, and the specifications for each. Use of airspace classes Each national aviation authority determines how it uses the ICAO classifications in its airspace design. In some countries, the rules are modified slightly to fit the airspace rules and air traffic services that existed before the ICAO standardisation. Australia Australia has adopted a civil airspace system based on the United States National Airspace System (NAS): Class A is used above FL 180 along the populated coastal areas, and above FL 245 elsewhere. Class B is not used. Class C is used in a 360° funnel shape in the Terminal Control Zones of the major international airports, extending up to the base of the Class A, generally at FL 180 over these airports. It also overlays Class D airspace at smaller airports. Class D is used for the Terminal Control Zones of medium-sized airports, extending from the surface up to AGL (depicted in MSL on a chart).