From Halopedia, the Halo wiki
Combat skins, also known as fighting suits or battle armor, were a type of body armor used by Forerunner combat personnel. The Forerunners had a classification system for combat skins, running from Class 1 to Class 18, and possibly above. According to 343 Guilty Spark, the human MJOLNIR Mark V is ranked on this scale as equal to a Class 2.
Combat skins ranged in shape and design from suits that mostly followed the wearer's figure in the manner of traditional battle armor, to much bulkier and more powerful variations that, while roughly humanoid in shape, were closer to vehicles than suits of armor. The war sphinx, a medium-sized fighting suit, was ten meters in height, capable of space flight, and equipped with weaponry powerful enough to destroy cities. The war sphinx's newer counterpart, the seeker, was yet more advanced. A combat skin could be laid directly over a Forerunner's personal armor, worn on everyday occasions; Forerunner warships stored such "combat wraps" in condensed form, allowing fighters to convert their armor into a heavier combat skin should the need arise.
One notable feature of combat skin was its ability to attune itself to best counter different forms of attack. For example, after optimizing to the Forerunner infantry weapons wielded by Blue Team, Ur-Didact was capable of ignoring numerous direct hits from the Spartans' weapons. Previously, automatic fire from all four members of Blue Team firing their MA5D assault rifles at once had no effect on the Didact, demonstrating the resiliency of a high-level combat skin. Warrior combat skin was also equipped with several offensive and defensive systems, including the ability to generate and manipulate various forms of constraint fields at will. The Didact's armor also possessed a number of energy attacks, including a directed energy blast and a powerful shockwave capable of stunning nearby enemies.
Combat skins were typically worn by Warrior-Servants, the rate tasked with warfare, but could be used by the dedicated military or security forces of the other the rates as well, such as Builder Security, the security force of the Builder rate. After the Flood threat intensified to critical levels, even civilians were forced to wear combat skins in high-risk areas.
Combat skins were comparable to, but far more advanced than, the UNSC MJOLNIR Powered Assault Armor. It is first mentioned by 343 Guilty Spark as he is leading John-117 through Installation 04's Library, wondering why he had brought "such ineffective weapons" to combat the Flood. He notes that the MJOLNIR is ill-suited to combating the Flood, and recommends that he upgrade to a Class 12 combat skin from the MJOLNIR, which is ranked on the Forerunner classification system as a Class 2. On the other hand, Guilty Spark also notes that the MJOLNIR will serve John well when the Flood begins converting Halo's atmosphere, commenting that he is a "good planner". Upon learning that there was a powered armor system six times more powerful than the MJOLNIR, John expressed enthusiasm at the opportunity to wear one.
The Terminals on Installation 00 indicate that other combat skin classifications existed, ranging from 1 to 4, and that there were other suits of a similar nature, such as the battle harness, with its own classification ranging from 1 to 6, and the platform interface skin. Eventually, all civilian personnel were required to wear at least a Class 8 combat skin unless in core areas, and all military personnel were required to wear at least a Class 12 combat skin.
During the Battle of Installation 00, Guilty Spark can be heard lamenting that John did not upgrade his combat skin. The fact that the Mark VI is more advanced than the Mark V that Guilty Spark had previously encountered and is still classed as a Class 2 may indicate that the upgrades were considered so marginal from a Forerunner standpoint that the MJOLNIR suit was still inferior to a Class 3 combat skin.
List of appearances