Fulfil sends the 3PL a customer shipment object - that represents an order. This allows merchants to split a single order from a customer to two or more shipments that can be fulfilled by different 3PLs.
Multi-warehouse Fulfillments
If you are a 3PL using this API, you will only be sent the part of the order (as a shipment) that you are responsible to ship.
Workflow
Customer shipments
Fulfil will check the inventory available at the 3PL virtual warehouse and allocate inventory. Only assigned (pending) shipments are available to fetch. If it's a backorder and there is no inventory available in Fulfil, such shipments remain in the waiting allocation. These shipments are not available over the API to fetch for shipping.
Partial customer shipments
Suppose you are shipping a shipment partially, update fulfil with the fulfilled items and the corresponding quantity. Fulfil will create another shipment automatically for the remaining items.
So the 3PL system should cancel any partials as Fulfil will manage the creation of another shipment for back-ordered items from a shipment, and they will have a separate shipment number.