What is difference of aluminum billet  air oil slip and hot top casting technology?

Attaching picture of cut view of billet. The main difference is the segregation zone, air & oil slip technology is better than hot top, it is good for  high precision and high sophisticated industry aluminum extrusion. At old stage, we used the hot top casting technology, for removing the segregation zone, we used a machine which called aluminum peeling machine to remove the segregation, but now, air slip technology can reduce this process to increase the product finished rate.