The Ceramic plates are what we use in the Military. They can take multiple hits as long as they aren't in the same place. The curvature is preference and comfort.
I have AR 500 steel plates which can take a hell of a lot more hits without penetration but the rounds fragment upon impact which creates a hazard so some craftsmanship is required to mitigate that.
The rating is pretty much saying it can take a hit up to .308. Which most military SAPI plates are rated up to which definitely serves a purpose when facing a force which carries AKs. I would jump all over the opportunity if it were at the right price.