Sorry Karen it is hard to put into words but yes you can redeem 50,000 points for $200.
However, once a person has redeemed 50,000 points for $200 on a cruise that person cannot redeem an additional 50,000 to make up to the magic 100,000 for $400 redemption level.
You redeem 50,000 then that is all you can do for that particular person on that particular cruise.
