Handling Split Orders and Partially completed orders
When orders are completed it marks the items as completed, but bases this only on item counts. It doesn't actually check exactly which items have been shipped.
When orders are exported to ShipStation the LineItemID is passed. This is the actual order item ID database ID. When orders are marked as completed (either full orders or split orders), it passes back the same LineItemID which can be used to identify exactly which items have been shipped in the order.
If only specific items are marked as completed this will support Split / Combined orders in ShipStation by allowing it to only mark the exact items.