Hi, 
After validating A purchase order, a picking order is created.  While validating it, if you have processed less products than the initial demand, the system will ask you if you want to create a backorder.
If you're interested in proceeding the remaining products, then Click on Create backorder. If not interested, Click no Backorder.

If you clicked on Create backorder ,then the system will validate the current shipment order and creates a new one with the remaining products.