This call is only relevant for a purchase which is tracking items and using target order quantity. It is used to split a draft purchase into two draft purchases, one of which will be ready for finalization.
For example, If the quantity of a purchase is 12, and the targetOrderQuantity is 14, then this call can split the purchase into two objects. The one with quantity 12 will have targetOrderQuantity 12, and the new purchase will have quantity 0, and targetOrderQuantity 2. This keeps track of the two unfulfilled tracked items.