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Tradeflow are usually divinded into various port-to-port voyages and port-to-port voyages are of 2 types: NAME_1 and Ballast. 
A NAME_1 voyage refers to a maritime journey undertaken by a vessel when it is carrying cargo. In other words, the ship is not empty but loaded with goods or freight that is being transported from one location to another.
A ballast voyage refers to a maritime journey undertaken by a vessel when it is empty. In other words, the ship is not carrying any cargo and is being transported from one location to another.