The DLC will take the form of a four-week event, and will be available to 'all players'. Developers, Ubisoft, have given some light on the Outbreak's backstory: "An infestation of unknown origin is attacking the small town of Truth or Consequences, New Mexico. The danger is so high that the authorities quarantined the entire town, trapping civilians and soldiers inside. Very little information is filtering out, but the only certainty is that the situation is dire."
Moreover, the DLC will bring a total of fifty unique items (available through Outbreak packs), which will not impact on the gameplay. In addition, Outbreak packs will be sold for 300 R6 Credits, though "a few other Outbreak rewards will be available through gameplay." Players will be able to get other packs through "other criteria to be disclosed at a later date." Also take note, that by logging in during the event, you'll get four free Outbreak packs.
Outbreak will launch on March 6th 2018. Furthermore, Ubisoft have announced a newly revised editions of Siege that will soon be available for sale, including the Advanced edition, that includes 10 additional Outbreak packs as a bonus.