So, an important part of armor is the different ratings.
They are (in order of protection):
(I-IIIA is usually soft armor, III-IV is usually plate.)
I - Barely counts as armor
II - Most pistol rounds
IIIA - All Pistol Rounds and some rifle rounds
III - Rifle Rounds
IV - Armor Piercing Rounds
So, the National Institute of Justice, which rates armor plates, defines a Level 3 plate as one that will stop 7.62×51 M80 ball and a Level 4 plate as one that will stop 30-06 M2 AP (NIJ 0101.06).
Some Level 3 plates are not able to stop M855 ball due to it’s mild-steel armor penetrating core. They weren’t designed with this threat in mind.
The "Special Threat" Plate is something not covered by the NIJ standards. But it was designed with M855 in mind.
The L210 drops M80 ball protection, but stops several AR-15 and AKM light armor-penetrating rounds, including multiple hits of M855.
So, the "2" in L210 means that it's technically closer to a Level 2 Plate in regards to M80 .308 and NIJ Standards.
But it is better than many Level 3 plates, when it comes to 5.56 and 7.62x39.
I included an image that shows it's ratings. And here's a breakdown from Hesco. (per round type, not combined):
5.56x45mm-M193: 3150 ft/s (6 hits)
7.62x39mm-M67: 2390 ft/s (6 hits)
5.56x45mm-M855/SS109: 3150 ft/s (6 hits)
7.62x39mm-M43 (Mild Steel Core): 2390 ft/s (3 hits)
7.62x39mm-API: 2390 ft/s (1 hit)