By default the shopping cart will allow any and all products to be added to the shopping basket. Checking this box will cause an error to be displayed if the customer adds a product that is NOT listed in inventory control.
This can be useful to prevent customers from ordering products that you have recently removed from your product listings but that may still be in their cached pages.
Please note, however, this feature will not allow the customer to purchase any current product until you have correctly entered it into the system.
Setting-up the not-in-inventory message