Assassin's Creed 4: Black Flag will notice that the game comes with a Uplay Passport, Ubisoft's version of an online pass. While the passport typically grants access to online sections in the game, in the case of Assassin's Creed 4 it also unlocks small pieces of social content.
Game Informer reports that the online pass locks away the game's fleet management system, in which players add vessels they board in the game to their fleet and send the ships out on missions to earn additional money. Players with second-hand copies of the game that don't plunk down $10 for a Uplay Passport are unable to manage their fleet, and since real-life friends are able to provide assistance to fleets, the management system is not an offline-only aspect of the game, even though it falls into the single-player campaign.
We've contacted Ubisoft for comment on the game's Passport and will update as we learn more.