D12 core's are lower density polypro core's. This low density PP Core's designed to have the lightweight strength, water resistance & stiffness of polypro core but with more flex like a PE Core.
D12 is 1.2lb PCF (Per Cubic Foot) in density) this means the core is lighter than Polypro Core. D12 Core cells are larger & less dense than PP. This is the choice of core's if you want a more flex for cold water or a performance board that has more flex. This core is also great for kids or women as it is much easier to control.