From p.46
Mountainous, desert/arid and forest/jungle territories that are not production centers (do not have a national flag) have limited infrastructure and can support only one air unit until more infrastructure airbases are built.
The unlimited airbase capacity of flagged territories:
Does it apply only to the owner?
Or anyone who controls the territory?
I.e. Can Japan base unlimited air units in the two Chinese Flagged territories in the Mountains, and the 1 flagged territory in the Forest?