One model has a build-up coat to reduce spauling or the spread of bullet fragments after it has hit the plate.
The Build Up coating makes the anti-spalling more resilient and keep more lead out of your chin and shoulders
They use PAXTON coating to protect the plate from rust. One is offered with a Base coating that will do just that, but the other that is offered, has a Build-Up coating, which is thicker, to prevent or minimize the fragments of the bullet stopped by the plate, from hitting your face and/or throat.
When a bullet hits Kevlar it deforms and is caught more or less staying in one piece. When a bullet hits hardened metal, especially dead on, it pretty much explodes into tiny fragments in all directions. Most likely not fatal but getting like 30 bee stings or worse in your throat after the panel stopped a bullet isn’t fun. So the more expensive one has a thicker coating on the front that is the companies proprietary form of truck bed liner that will help catch the small fragments and prevent them from leaving the panel. All the ar500 armor companies list their base models as having a base coat of spall liner. It might help a little but that’s more to waterproof the plate and make it look better. It does help a little not a lot. You probably want the spall liner. They put more of the same coating on the front. It does add some thickness and some weight but better than spall to the throat. That steel plate will probably take 100s to thousands of rounds. The spall lining would probably be gone by 30. Granted if I’m standing around and letting someone hit me after like 2, pretty sure they’re already shooting at my corpse