You are mixing up Qpoints with Qcredits and Qmiles

Qpoints is what you get for every flight.You need them to renew your status tier. They have no other usecase.
Qmiles you also get for every flight but you can use them to upgrade other flights as well as to buy redemption tickets or even use them at the store to buy stuff.
Qcredits are provided to members once they reach a specific tier or renew it. They can be used to upgrade flights or get extra baggage/lounge access.
Qcredits don't care about the booking class while with Qmiles it the upgrade price is different depending on the booking class.