In modern common extrusion dies, if you want to produce hollow profiles, you must use a split die, which consists of an upper die and a lower die. The upper die is generally used for guiding and positioning the hollow part of the hollow profile, and the lower die is the outer surface working belt and the empty knife. Most non-hollow profiles use flat dies. Generally speaking, only one die includes guide and working belt. Of course, there are some special circumstances, and here is just a general explanation.