The stock card can be linked to the product using a recipe. Thanks to the recipe, the entered ingredients are deducted from the stock at the moment when the product is ordered in the POS application. To set up the recipe, it is first necessary to have the product created and the stock cards of the raw materials it contains (if the product is named the same as the stock card and has the same unit, you can create it directly when creating the stock card). For example, if you are going to stock a can of Coca Cola in pieces, you also need to have the recipe in pieces. When selling 5 pcs of Coca-Cola cans, 5 pcs are automatically deducted from the warehouse.
You will learn how to set the recipes correctly in this guide: https://help.storyous.com/hc/en-us/articles/360000343489-RECIPES