Codes to download "Specialization Priority Alpha" are currently being emailed to anyone that participated in Halo 4's multiplayer prior to November 20, Kotaku reports. The DLC acts as a key that unlocks Engineer, Tracker, Rogue, Stalker, Patherfinder and Operator Specializations, all of which grant unique armor and abilities after Level 50.
For anyone that doesn't meet that criteria, well, you can always make your own unique armor out of cardboard and duct tape. Other players won't be able to see it, granted, but you'll know. And, in the end, isn't that the important thing?