'Milk Run' is a special form of direct transport.
The term comes from the image of the traditional milk boy who delivers a full bottle of milk only when he finds an empty one and takes it with him. The reason for this was that there was never too much milk in the house and then the milk could get bad.
The "milk run" is therefore a transport process on a fixed route, usually with a fixed time and fixed quantities. The collection times are predefined and the recipient is directly reached without using a transshipment terminal. The idea is that only the quantity that has been consumed is replenished.