Staterooms are still on a first come first serve basis. You do have the option of putting $50 down towards your stateroom.
It allows Carnival Cruise to know that you are taking the cruise, and they build a list which partially reserves your stateroom.
Your $50 will go towards your deposit but if full deposit is not paid by the deposit due date of Feb. 1, 2025. Your $50 will be refunded to you and you lose the reservation.
Your funds will remain on group level until full deposit is paid.