Instance: You might have an e-commerce Web-site with products webpages for various sizes of a particular t-shirt.   You specify the principal URL since the internet search engine can have problems identifying which a person should be the principal URL.  The next tutorial will teach you how to enable Canonical https://www.presta-addons-modules.com/en/seo/prestashop-seo-automatic-hreflang-tags-canonical-tags-on-all-pages.html