Halo 4 has introduced a new feature that will drastically alter the Xbox Live multi-player landscape. Players are now able to focus on eight different "Specializations", each with their own class specific bonuses.
The eight "Specializations" are: Engineer, Rogue, Tracker, Pathfinder, Stalker, Operator, Wetworks, and Pioneer, all offer players highly specialized routes to take for their multi-player experiences. Two of the classes will be available immediately when the game is released on November 6. Pioneer, which grants XP at a much faster rate when the buff is active, and Wetworks, which allows a quieter run and use of Promethean gear without drawback, will be the two classes available on day 1.
The other six classes will be made available "over time" after the release date. All of these character classes, and the ten levels each one is capped at, are filled with a ton of aesthetic tweaks (armor mods, helmet mods, skins, armor sets, etc.) that allow you to customize your Spartan to truly set him apart.
Via:Halo Waypoint, Joystiq
