Positioning flights by company aircrew don't qualify as confirmed, reserved seat
Really? In a couple of airlines with which I am very familiar, positioning crew (to operate a flight or who are returning to base after an operation) are booked as "must go" status. The space is confirmed and pity help any ground staff who offload crew in those circumstances. The required number of seats are blocked with the saleable inventory reduced accordingly. It is therefore possible to still overbook a flight based on the reduced inventory.