These objects are individual delivery moments in a purchase order.
A single purchase order can be delivered in multiple parts. Each registered delivery moment instantiates a new delivery object. For an aggregated view of all deliveries in a purchase order, you'll want to retrieve aggregated delivery objects.
Accepting a delivery in Apicbase is already sufficient to create a delivery object, but as long as it has not been saved yet, it will remain in a draft state. While in this state, most data can still change. Saving a delivery will commit its updates to inventory and set its draft state to false.