If Check Stock? is enabled, your cart system will not allow shoppers to add more items to their cart than are available in stock.
If Show Products that are out of Stock is enabled, products that are currently not in Stock (zero inventory by default) are displayed to shoppers.* Otherwise such products are hidden from shoppers.
You can also set the cart’s Out of Stock threshold quantity to a value greater than zero. The cart will then treat all items as not in stock when the stock quantity reaches this threshold.
If you have “Show Products that are out of Stock” checked, and a shopper tries to add a product with 0 inventory to their cart, the system will let them know that there are not enough items currently in stock to fill the order.