There are different classification standards for this product, the details as follows: Based on the shape,one is T29,and the other is T27; Based on the usage,one is for metal,and the other is for Inox; Based on the material,one is made of Aluminum oxide,and the other is made of Zirconium; So should you are interested in the flap disc,pls tell me your detailed requirment,So that I can offer you a accuary quotation for your reference.