How do trade-in value cruise certificates work?

A lot of cruise certificates will say a cabin type offer or trade-in Dollar Amount. On the back, it will have a list of 20-40 sailings where you can use that certificate for a specific cabin offer. That means if you were to choose a sailing, not on the certificate, the cruise line would look at the price of the sailing you chose and deduct the dollar amount of the certificate from the sailing's price.