Sorry, I am fairly new to this game, could you please explain how this works?
AA is zone based and allows free stopovers at the North American gateway only.
So, HKT-YVR is Asia 2 to North America. However, you could tell AA you're interested in booking HKT-JFK for example, which would be the same mileage, and route yourself HKT-HKG-YVR-JFK (if allowed by the CX fare), stopping over in YVR for months.
Essentially, you now have a free YVR-JFK flight (potentially in CX J, since they fly that route) for future use.
That's an example, you could also book a AA/AS flight YVR-XXX, if allowed by the CX fare.