Combat skin

Combat Skin is a Forerunner term used to talk about body armor. In has a classification system from Class 1 to at least Class 12. There may be more higher classes after that 12. MJOLNIR armour is ranked on this scale as Class 2.

Summary
Not much is known about Combat Skin. It first comes up in Halo 2 on the Library (Level) when 343 Guilty Spark wonders why the Master Chief had brought "such ineffective weapons" to combat the Flood, and recommends that the Chief upgrades from the MJOLNIR armour, which, according to some Forerunner ranking system, the criteria of which is not gone into, is only Class 2, to a Class 12 Combat Skin, which 343 says is necessary to combat the Flood.

Here 343 is referring to Combat Skin as a type of armour where the MJOLNIR armour is Class 2. However, later 343 talks about the Flood changing the atmosphere, he says "Your Environment Suit should serve you well..." implying that the MJOLNIR armour in Forerunner terms is an Environment Suit.

First Theory
343 Guilty Spark is referring to the MJOLNIR armour which can be called either, Combat Skin or an Environment Suit.

Second Theory
The differing nomenclature suggests that the two items 343 refers to are different, i.e. your armour is what he refers to as the 'Environment Suit', and your 'Combat Skin' is the Chief's flesh and body.

Which brings up an intesting idea of the Forerunner. There is no evidence that speaks of any physical Forerunner form, save a Forerunner hand print that may just be a computer pattern. Therefore the forerunner could have no body and lead a life much like a parasite. So it's possible that the Forerunner is an entity inside the Master Chief's body.

Chief certainly seems to know how to use Forerunner tech even though he's not supposed to have come across it before, and when 343 Guilty Spark first sees Master Cheif he seemed to recognise him. Perhaps 343 didn't recognise the Chief's physical form but something IN the Chief.

This theory brings up other possibilities that maybe the Flood are a type of Forerunner either a branch on the evolutionary tree or some kind of gentic enhancement of the orginial Forerunner. This could go the other way of course maybe the Forerunner is the spawn of the Flood, a weapons program gone wrong.

It could also explain why all the doors on Halo are different sizes, because depending on which being the Forerunners were inhabiting, different sized portals may have been necessary. The idea that GS isn't actually talking to the Chief could account for the perception that GS thinks the Chief has set off Halo before - he doesn't think the Chief has at all, as he is actually addressing the Forerunner in John-117's head, who have/has set Halo off before, and having done so were forced to inhabit the forms of the very creatures they sought to contain.