How to decide which aerodromes should have Electronic Terrain and Obstacle Data?
An aerodrome or airdrome is a location from which aircraft flight operations take place, regardless of whether they involve cargo, passengers, or military. Aerodromes include small general aviation airfields, large commercial airports, and military airbases. 


  An airport is an aerodrome with extended facilities, mostly for commercial air transport. Airports often have facilities to store and maintain aircraft, and a control tower. An airport consists of a landing area, which comprises an aerially accessible open space including at least one operationally active surface such as a runway for a plane to take off or a helipad, and often includes adjacent utility buildings such as control towers, hangars and terminals. Larger airports may have fixed-base operator services, airport aprons, taxiway bridges, air traffic control centers, passenger facilities such as restaurants and lounges, and emergency services.


With this definitions of aerodrome and airport from Wikipedia, we understand that all airports are aerodrome but not all aerodromes are airport i.e. airport is a subset of an aerodrome.

The Civil Aviation Safety Authority under CASR part 139 classifies the Aerodrome into below categories:


Certified Aerodromes
Registered Aerodromes  
Other Aerodromes - more than 9 but not more than 30 passengers
Other Aerodromes - operations under proposed CASR part 135