A few SEO’s have written that the x-default is only to be used when you have a home page that has a country selector and should only be used on this page.
Clarification: This is only partially true. There are two specific applications of the x-default. The first applies to sites with a selector like FedEx or Ikea. These sites present a splash page with a language and/or country selector asking the visitor to choose which location and/or language version of the site they want to visit. Since this page does not target any specific language or country the x-default tells search engines to present this page in any market that does not have an assigned page.
Where the SEO’s are incorrect is how to handle older and large multinationals, especially those in the United States, which often use the main dot com site as both their global site and their US site. In this situation, they should also use the x-default. A good example is Cisco that does not have a dedicated US version of their site. Since these pages do double duty, to make them both the preferred “rest of the world page and set it for the US, they need to add a double entry to your hreflang to set the same page as both x-default and US English similar to the example below.
<link rel=”alternate” hreflang=”x-default” href=”https://www.mysite.com”/>
<link rel=”alternate” hreflang=”en-us” href=”https://www.mysite.com.com”/>
<link rel=”alternate” hreflang=”pt-br” href=”https://www.mysite.com/pt-br/index.html”/>