From Halopedia, the Halo wiki
The MJOLNIR CQC variant was the first of its kind to aid and improve the Spartans' effectiveness in close quarters combat. The armor is designed for tactical, short range battles in urban areas, and as such the CQC variant offers an improved field of visibility and heavy ballistic protection. In addition, numerous sections of the helmet supports open-port conversion - this allows the user to add any type of compatible module to the helmet to further improve functionality, allowing a Spartan to adapt to changing mission conditions on the fly. The CQC's heavy pauldrons allow a soldier to swiftly and lethally close the gap between an enemy combatant despite being under fire. The CQC can be utilized for both military operations on urban terrain (MOUT) and boarding operations aboard ships which are comprised of tight, enclosed spaces.
By the early 2540's, it was field-tested at the Special Warfare Center Essen, in Deutschland, and integrating feedback gathered from the Special Warfare Center Seongnam, in Korea. The CQC's Mark IV variant entered service in 2548 and saw use among SPARTAN-II and SPARTAN-III soldiers. The CQC variant was further upgraded for use with the MJOLNIR Mark V in 2551.
Despite being surpassed by the CQB in overall integration, the CQC variant still retained popularity well after its production ceased. Many still prefer CQC's less traditional open-port conversion system to allow ad-hoc modification. Some Spartans continue to use CQC instead of the newer CQB because of the versatility and mobility the CQC provides.
Similar to the CQB in Halo 3, the CQC helmet is unlocked by default in the Armory. The other armor types are unlockable in the game's purchase system via the use of cR. In Halo: The Master Chief Collection, the CQC set is attainable in the Season 1: Noble progression system.
The helmet's design bears similar resemblance to the Mark VI helmet and the limited production helmet on the Mark IV. Two attachments are available for the helmet: the first adds a Chemical, Biological, Radiological, and Nuclear (CBRN) Module for operations in hazard zones. The second adds an Up-Armor plate with a Hardened Uplink Module on the right side (UA/HUL) for MILINT acquisition. The CQC pauldrons are rectangular and flat in shape, and have two smaller "squares" on the top and bottom. These shoulders go down to about the elbow. They can be seen at the rank of Major, but require the rank of Lt. Colonel to be bought.
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