What exactly does "Commander" mean? Synonym of First Officer?
I know nothing about Cathay Pacific ranks/titles, but the plain words seem pretty logical.
The Captain, as Pilot In Command, is legally responsible for the entire flight. But who is actually in command while the Captain is required to be resting on long flights?
You don’t want the two most senior pilots resting at the same time; it would be unsafe to put a junior pilot in command for half the flight. So, you designate the second most senior pilot, who is still technically a First Officer, to be in command while the Captain is resting.