If you do not have an invoice yet but you want to update your inventory with items received from a vendor, you can select the Waiting on Bill check box. This lets you update your inventory records with the items you have received, while you wait for the invoice. When you receive the invoice from the vendor, open this receipt of items and enter the invoice number, making sure you check the prices and such.
Note: Until you enter an invoice number for this receipt of items, you will not be able to pay the vendor for the items.