Search engine optimization implies "Seo". It represents a list of tactics and greatest methods aimed at improving upon the visibility of an internet site on serps.   You specify the primary URL because the search engine could have problem determining which just one should be the primary URL.  The following https://www.presta-addons-modules.com/en/seo/prestashop-seo-automatic-hreflang-tags-canonical-tags-on-all-pages.html