In order to avoid confusion with the classic stock management feature from version 1.4 of PrestaShop and see the possibilities offered by the advanced management feature introduced with version 1.5, you should picture two distinct concepts: product quantity available for sale, and physical products.
Product quantity available for sale
This is the same as the stock management feature from PrestaShop 1.4.x. It is the product quantity that is displayed in the shop for each product and product combination. This is the quantity that defines whether the product can be ordered or not (unless the "Allow ordering of out-of-stock products" option is enabled). That quantity can be manually changed for each product and product combination.
Consequently, what used to be called "stock" in PrestaShop 1.4.x is called "quantity of product available for sale" since PrestaShop 1.5.x.
Product stocks (physically stored)
This is the physical management of the stored products from a warehouse (or more). This is the new concept introduced as "stock" in PrestaShop 1.5.x.
Real stock is therefore made of stock that is physically available in a warehouse, to which we add the quantity presently ordered from suppliers, and from which we subtract the quantity presently ordered by clients and which have not yet been shipped.
Using the new stock management feature
Do I have to use the new stock management feature?
There is no obligation to use the new stock management feature, just as there is no obligation to use the "quantity available for sale" management feature.
The advanced stock management feature, or even the standard stock management feature and your warehouses, is independent from the multistore feature. Consequently, no matter which shop you are administrating in the PrestaShop back-office, when you use the "Stock" menu, you are always managing the stock in a global way.
I do not want to change anything to my settings in PrestaShop 1.4.x/15.x. What should I do?
If you would rather not use the new advanced stock management feature from PrestaShop 1.5, and simply are satisfied with the way PrestaShop 1.4 handles product quantity management, you just have to enable the old-style stock management manager, and not the advanced stock management feature: go to the "Products" preference page, scroll down to the "Products stock" section, and choose "Yes" for the "Enable stock management" option, leaving the "Enable advanced stock management" option set to "No".
The "available for sale quantity" management feature is now centralized in the "Quantities" tab from the product sheet: create a new product or edit an existing one, and the tab is available on the left, among the other tabs.
Does the new stock management feature apply to my needs?
The new stock management feature enables you to manage a stock of products. This feature applies to your business if: