If you use an external system for transportation planning, a record of deliveries to be transported is selected in the SAP system and sent via the interface to the planning system. There the deliveries are combined into shipments according to specific optimization criteria. The shipments that were generated are confirmed in the SAP system which in turn triggers generation of the shipment documents. In order for the shipment documents to be updated properly, master data (customer address or goods acceptance times, for example) must match in both systems.
Grouping and changing of shipments occur systematically in the transportation planning system. All further shipment processing activities (printing or reporting start of shipment, for example) are treated like regular document processing in the SAP system.