Transportation planning in SAP involves creating shipment documents to plan transportation of goods from vendors to customers. Key functions include transportation planning, shipment cost calculation, and shipment tracking. Shipment documents combine deliveries into shipments, specify transportation stages and legs, and generate outputs like shipping manifests. Shipment cost processing calculates transportation costs using pricing procedures and conditions. EDI is used to share information with external logistics systems using idocs for activities like advanced shipment notifications and shipping confirmations.