This feature is for you who bought stocks but haven't received the goods. So the inventory level has not been added by new stocks.
Step 1. Create Purchase Order on Buy menu then click shipping
Select "by Delivery"
Step 2. If you See the invoice, there will be delivery section. Which is still have 'Pending' status.
Then click on the 'Receive' button to process the coming goods.
Step 3. After clicking the 'Receive' button there will appear the 'In Delivery Form'. Fill in the 'QTY' column which goods are received and click 'Save' button.
Afterwards, if you look back to the invoice, the status of the delivery will changed to 'Complete'.
But if there still some un-coming goods, the status will show 'Partial'.