Combat skin

Combat Skin is a Forerunner term relating to body armor. The Forerunners had a classification system for combat skins, running from Class 1 to 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.

Description
Very little is known about the Forerunner Combat Skin, other than that it was used by combat personnel and was 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 the Library, wondering why he had brought "such ineffective weapons" to combat 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, and notes that the MJOLNIR is ill-suited to combating the Flood. 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," perhaps indicating that this was a function that a Combat Skin lacked. 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 at least the early years of the Forerunner-Flood War, Forerunner ground troops used Combat Skins that, while certainly powerful, lacked energy shields, leaving their wearers extremely vulnerable to Flood infection. It is likely that, rather than representing the peak performance of the technology, it is indicative that the Forerunners were woefully unprepared for the Flood onslaught, and were initially slow to adapt their equipment and technology to fighting a purely biological foe.

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 Combat Skin class system is based on another factor other than combat performance, or that despite it's upgrades it was still inferior to a Class 3 Combat Skin.

Speculation
Because so little is mentioned about combat skins, there is much speculation about exactly what the combat skin rating indicates. The obvious interpretation is that the combat skin ranking system measures raw armour power, and that a class 12 combat skin is 10 ranks higher in protection and power than a class 2, but if the Forerunner were able to use a suit at least six times more powerful than the Chief's, it's a wonder they had such trouble with the Flood, unless the Flood made use of such 'Combat Skin' themselves, such as an Elite Combat Form using a personal energy shield. This would mean that both forces would be equal in suit strength, as well as weapons and vehicles, while the Flood would most certainly outnumber the Forerunner. The most likely scenario is that in the Forerunner-Flood War, the Flood infestation was billions of times larger in scale, which would mean that despite the Forerunner's near invincible armor, they may simply have been outnumbered.

Armor Class
Another interpretation is that the class system does not measure raw power, but rather classifies armor by it's role. Later in the game, as 343 Guilty Spark comments on the Flood's alteration of the atmosphere, he notes that the Master Chief's Environment Suit should serve him well. This may indicate that Combat Skin with a Class 2 designation are designed for use as environment suits in hazardous terrain, rather than as effective armor to be used in combat against the Forerunner's enemies. There is a discrepancy to this assertion, however, as one of Halo 3's terminals makes mention of higher classes of Combat Skin that were vulnerable or ineffective against the Flood.

Environment/Combat Suit
Another theory states that a combat skin may also be an environment suit. A modern day analogy would be a HAZMAT suit, which is an environment suit, but has no further use in combat, while a suit of Kevlar provides physical protection but no environmental protection. John-117's "combat skin" also functions as an "environment suit"; this can overlap with other theories, as a useful environment suit may lack in either pure power or suitability for its role in combat. Quite possibly, John-117's "combat skin" may be half environmental and half battle armor.

Shielding Theory
A theory that states "combat skin" does not refer to armor at all, but rather shielding; the disparity in power between the Master Chief's shields in comparison to that of the Forerunners is obvious. This is again compatible with either the power or role theory, as Class 12 shielding may be powerful enough to shrug off any Flood attack, or specifically designed to handle the damage from the Flood. Some have specified that if you raise your shield level to maximum in Halo 3's settings, you would have something close to a Class 12 Combat Skin, although this will never be known as correct or wrong since the actual power of the Class 12 Combat Skin has never been witnessed.

Armor Ability Stacking
Another possible theory is stacking. If a normal HAZMAT Suit has a ranking of 1E (1 Environment Point) and a high class KEVLAR Suit has a ranking of 1SA (1 Shield & Armor point). Mk VI (as stated above, and shown in game) is a 1E + 1SA, giving it a rank of 2ESA (ESA being a single class rank in the forerunner system), giving John's Mark VI MJOLNIR a Forerunner rank of Class-2. With this being said, 343 stated that John should wear at least a Class-12 Combat Skin, which would be the equivalent to 6 post-modern HAZMAT Suits and 6 post-modern KEVLAR Suits or another 5 layers of MJOLNIR armor.