As many sites have a global home page with a country or language selector that they want to be the default "Home Page" Google launched the X-Default to let you indicate which page this should be. Originally designed for this home page you can use the x-default tag on any version of the site that is in effect the default or master version of the site.
The syntax of the x-default is the same as any language setting <link rel="alternate" href="http://example.com/" hreflang="x-default" />
Setting x-default in HREFBuilder
Step 1 - Select the country language that you want to set to X-default and click the "Edit File" option for that entry
Step 2 - In the Country Language Selector, click the pull down and select the X-Default option.
Step 3 - Click the Blue up or down arrows to go to teh top or bottom of the page and click update.
Considerations & Options
1. Choose the version of the site you want to be the global representation of your company.
2. If the x-default version of the site also represents a country like the US and is in US currency you can also set it for the US. To do this you have to download the dataset which you can do from teh Edit Screen" and reload as that country. If you have already set the US as the global English or for the US you make that equal to x-default. When you upload the dataset please change the name to x-default or the country to prevent it from being imported into the exiting entry.