It is basically like a shopping list that consists of all the physical assets of a particular product or system you work on. This exists as a hierarchically structured blueprint of products and elements that the project mandates to achieve or the outcomes that the project will yield. PBS starts with the end product from the top, pursued by subcategories of elements correlated to the product. The composition of the product breakdown structure is more likely than the structure of the work breakdown structure. https://www.quick-devis.com/en/work-breakdown-structure-in-project-management-matters/