The stock tab will only show up if you have Payment Tracking enabled in the component's options. It is only applicable for Buy now, shopping cart and subscription buttons.
Track stock for this item - Set it to yes to track stock.
Quantity in stock - Set the number of items in stock.
Button's out of stock message - Message displayed instead of the button's image to notify the user the item is out of stock.
Out of stock notification - Message displayed to user is they try to purchase an item that does not have enough of the selected quantity.