Instance: You might have an e-commerce Web-site with product or service internet pages for various measurements of a specific t-shirt.   You specify the primary URL because the internet search engine could possibly have difficulty determining which one particular need to be the primary URL.  The following tutorial will provide https://www.presta-addons-modules.com/en/seo/prestashop-seo-automatic-hreflang-tags-canonical-tags-on-all-pages.html