APEX-dressing stones are vitrified bonded. The use of abrasive material as silicon carbide or precious corundum depends on the purpose.
Model A: it is used for the dressing of grinding wheels. Made of silicon carbide. Its colur is blue, grew to black. The coarse grain is hard bonded to achieve durability.
Model B: the silicon carbide is soft bonded to achieve durability. It is used to work on large surfaces like concrete or stone.
Model EK: this one is made of percious corundum, colour pink. Its combined composition is coarse grain and on the other side fine grain and it is mainly used to work on tiles. Please note that this side with the fine grain is intended to work on the glazed tile. The other side made of coarse grain is intended to work on the non-glazed tile.
More details here