Assassin's Creed 4: Black Flag players are required to own a Uplay passport for the game if they want to manage ships in their fleet and send them out on missions.
The new policy was discovered by GameInformer, which reports that certain segments of gameplay can only be accessed if the player enters the unique code included with new copies of the game.
In effect this means that those who purchase the game second-hand, or borrow it from a friend, will not be able to experience one of the core elements of the Assassin's Creed 4 single-player experience. The unlockable feature is said to be similar to sending assassins out on missions in Assassin's Creed Brotherhood.
It is likely that codes for unlocking the content will be available to purchase through Xbox Live, PlayStation Network and on PC.
In CVG's Assassin's Creed 4: Black Flag review, our critic calls the Ubisoft sequel "a return to form for the franchise", issuing a 9/10 score.