Is there some sort of pattern to determine when Mileage Plus miles can be used to book non-Star Alliance subsidiaries/partners of Star Alliance carriers?
I (pleasantly) discovered that Mileage Plus miles can be used to book Ibex Airlines, who cooperates/codeshares with ANA but isn't even majority owned by ANA. But Mileage Plus miles can't be used to book AirDo, even though AirDo is majority owned by ANA.
As another example, for Air China's majority-owned subsidiaries, Mileage Plus miles can be used on Dalian Airlines, but not Air Macau nor Tibet Airlines.
Is it just per the whims of the Star Alliance parent?