This feature is meant for you who bought stocks but haven't received the goods. So the inventory level has not been added by new stocks.
How will you be able to do so? Just follow the sequence as illustrated below :
Make purchase orders as usual, but right after choosing 'Pay' button click on 'MORE' button so there will appear like the illustration below and choose 'By Delivery' on delivery section then 'Ok'
If you look to the invoice, there is delivery section. On which is still have 'Pending' status.
Then click on the 'Receive' button to process the coming goods.
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 uncoming goods, the status will show 'Partial'