The areas are defined as follows:

• Sea area A1.
  The sea area within radio telephone coverage of at least one coastal VHF station in which continuous digital selective calling (DSC) alerting is available. Basically this area extends up to 50 nautical miles from the coastal station.
• Sea area A2.
  This is the sea area, excluding area A1 within the radio telephone coverage of at least one MF coast station in which DSC alerting is available. This area typically extends up to 150 nautical miles off shore, but would exclude any A1 areas. In practice, satisfactory coverage can often be achieved out to 400 nautical miles off shore.
• Sea area A3.
  This is the sea area, excluding areas A1 and A2, within the coverage of an INMARAT geostationary satellite in which continuous DSC alerting is available. The area lies between the latitudes of 70° S and 70° N but excludes areas A1 and A2.
• Sea area A4.
  This is the area outside of areas A1, A2 and A3. It is essentially the polar regions north of 70° N and south of 70° S. It excludes all other areas.