Originally Posted by Wikipedia
The armor plates are specifically designed for combat with the Covenant. It resists the Covenant's plasma-based weaponry extremely well; a SPARTAN soldier wearing this armor can survive approximately three times as much damage from plasma weapons than an ordinary human marine could. The armor also provides basic protection from human ballistic projectiles, although only to a slightly higher degree as regular marine combat armor. The suit itself is practically indestructible and is capable of sustaining tremendous damage and still be able to function, but even though the suit will continue to operate, the human inside can still be injured or killed by a penetrating round or by sufficient plasma impacts to the suit's outer shell.