From Halopedia, the Halo wiki
Armored Marines are a heavily armored kind of Marine unit, boasting an extremely heavy variant of the UNSC Marine Corps Battle Dress Uniform.
The Armored Marines are classed as core infantry, and perform well against infantry, vehicles and aircraft. If this unit is in fatal/sever condition, if this unit is guarding another unit while it's be healed, this unit will also receive healing regardless if the unit it's guarding is full health or not.
List of appearances