Translations:User Manual/Administration Interface/Product details/21/en

If you leave the stock level field empty or don't add a stocklevel/warehouse combination for that product, it's considerd to be available for an unlimited number of times. This is especially useful for virtual goods like downloads.
 * Stock level (optional ) : The stock level itself can be any integer number and products are considered in stock, when the number is above zero. The stock level will be decreased by every bought product. If the value reaches zero, the product can't be bought by customers any more. It may be possible that the number can get less than zero if more than one customer buys the product simultaniously, because the stock level is decreased only when the payment was successful.