It refers to a return transportation phenomenon in which materials are transported from the place of production to the place of sale, and then returned from the place of sale to the place of production. There are two forms of reverse transportation: one is that the same material is shipped from the place of sale back to the place of production or transshipment; the other is that the same kind of material that can be produced and consumed in place A is transferred to place A from place B, and Ship to C