A flat URL looks like site.com/product-name, while a hierarchical one looks like site.com/category/product-name. Both work, but a simple structure is easier for users and search engines to read. However, using categories can help organize content better, especially if the site has many products. It shows how pages are related, which can help with ranking. The important thing is to stay consistent and keep URLs clean and short. Changing URLs often can cause issues. Which structure do you feel is easier to manage in the long run?