When you spend money with the credit card, the points go in as "choices". You can then use only these "choices" to redeem on your credit card statement. So if you already have some miles in your MP account and were thinking about using those they are off limits. For your example of $450 and assuming you get 1 "choice" for every dollar you spend, then you would need to spend $45,000 on the credit card to have 45,000 "choice" points to redeem for the $450. I like the choices program for this reason that you can redeem fewer miles for a ticket, plus you can earn miles on that flight.

Yes you can log into your account you can see a specific balance for your "choices". These "choices" can be used like regular miles and are in your total mile balance, but in order to redeem the "choices" you have to buy the ticket on your credit card and then redeem them from your balance.

Hope this helps. I know it is a little confusing