The detail was not fleshed out by Ubisoft and was instead revealed (perhaps by accident?) by Microsoft on its product page on Xbox.com for the upcoming game.
Online co-op for two to four players is mentioned, although there are no details about what that actually entails. We speculate that the game may offer a tactical multiplayer experience akin to Ubisoft's other upcoming title Ghost Recon: Future Soldier.
According to the product page, Assassin's Creed 3 is also set to offer standard 8-player competitive multiplayer, which will undoubtedly be along the lines of the multiplayer modes offered in Assassin's Creed: Revelations.
We'll post any updates on the game's multiplayer mode once more details become available. The game is set for release later this year.