S/O is short for shipping order, which means shipping order.

A document issued by a shipping company that has accepted a shipper's application for shipment and issued to the shipper to order the captain to ship the goods on board.

The S/O is given by the shipping company. After booking space with the shipping company, the shipping forwarder will receive an S/O indicating that the shipping company has allocated space and confirmed the booking. You can pick up and pack the boxes according to the S/O's instructions.

Therefore, S/O is sometimes called a packing list because as long as you receive the S/O from the shipping company, you can arrange for towing, container picking, container loading, customs declaration, and port collection.