There are 3 categories of airports: Local, National and International. For each of this categories revenue flights will be available only for a specific type of aircraft.In other words:

1. Local airports (about 6200) will show revenue flights for aircrafts of type A,B,C,D only.
2. National airports (about 1600) will show revenue flights for aircrafts of type E,F,G only.
3. International airports (about 200) will show revenue flights for aircrafts of type H,J only.

This means that in a local airport revenue flights won’t be available for a Boeing 707, so as in an international airport there won’t be revenue flights available for a Diamond DA20 Katana. Should one of your aircraft be sitting in a wrong airport it can be easily transferred with a ferry flight. Ferry flights between airports of different category are allowed.