Fatigues are part of the Battle Dress Uniform worn by military personnel of the UNSCDF during combat duties. Rather than full-body combat armor, like those worn by the ODSTs, regular fatigues are relatively simple cloth garments, with the benefit of flexibility and light weight, though at the cost of protection from enemy fire.
Though it provides very little actual protection, it can be covered with ballistic armor plating, such as the M52B Body Armor, and can also include camouflage to allow the wearer to blend into their environments. Known patterns are a grey "urban" color, a mottled green "jungle/forest" camouflage, and a light grey/white "snow" camouflage. While not nearly as effective as Covenant active camouflage, or Spartan-III SPI suits, it provides users with a fairly decent amount of concealment from enemy forces.
Fatigues are worn on missions where the added protection of heavier armor is a disadvantage, such as difficult terrain or for stealth missions. The clatter of armor may alert previously oblivious enemies to their approach, and the weight and bulkiness of the armor may inhibit the wearer's combat performance.
Likely to consist of normal cloth, rather than heavy armor plating, fatigues seem to provide little actual protection, especially from Covenant Directed-Energy Weapons, which can melt and burn through most materials easily anyway. Advances in fabric materials and manufacture may allow the cloth a Kevlar-like resistance to projectile weapons, or high durability, but its chief advantage lies in its light weight and easy of movement.
Halo: Combat Evolved is the only known game to feature UNSC Marines wearing only regular fatigues, without the overlaid armor plating, especially in the mission 343 Guilty Spark. This does not alter their abilities or statistics, and non-armored Marines can take as much damage as armored Marines.