Understand The Different Areas of the Site You Can Set Up Tests On
By default the test runs on the page you made it on, but multi-page, site-wide or special locations are possible. You have two options in site area:
Include an area of the site
Exclude an area of the site
Then you can use one of the three option to include or exclude pages of your site:
By Page URL
By Query Sting
By Tagged pages (not available at all plan levels)
Site area can be used for site wide testing, targeting specific pages like a product or category page that has something special in the URL or using pagetags to signal backend user plans and target on that.
When interested in multi website testing (multiple domains) and running an experiment on multiple domains. Use this area to define all the domains involved and make sure they are added in the Project Settings as well.