These days, body armor can be made from multiple materials, however, when it comes to hard armor plates, the most common materials are steel, ceramics, and UHMWPE. Level III armor can be made from all three materials, however, level III+ most commonly comes in the form of a steel plate or ceramic plate.
Steel plates have an inherent problem of spalling, which is when the bullet or its pieces can ricochets off the plate and hit the person wearing body armor, or the people around them. To solve this problem, steel plates have a rubberized strike face, which can stop slapping for a couple of rounds, however, after a couple of hits, it can be removed. Moreover, level III+ body armor plates are designed to stop green tip rounds, which have a partial steel core. Steel core ammunition does not break like normal bullets on impact and can be much more dangerous if it is deflected off a steel plate.
However, level III+ body armor plates are also made from Ceramics, which are much safer, and can take multiple hits.
Level III+ balances the highest protection possible at the lowest weight possible. Staying lighter enables the operator to move as swiftly and efficiently as they can. Quick maneuverability is crucial for engaging and disengaging from combatants, adapting to changing situations, and taking cover in unpredictable environments.