The Bonus Ball is the seventh number that is drawn from the remaining numbers after all the main six numbers have come out. That also means the Bonus Ball is unique and cannot repeat any of the main numbers. The Bonus Ball is designed to give you an additional chance to match the remaining number and win a great prize - the second-tier prize.
You don't need to pick the seventh number on your bet slip, any of your chosen numbers can be used to match the Bonus Ball.