This feature/rule/setting is pretty straight forward, whoever visits your website through a custom link, will see the Content Group (version of your website) that belongs to that specific link. They will land on your Home page and see the version of your website linked to that link. (Unless you are using an inner page/post custom link)
Use this feature carefully as these links can be shared everywhere.
You could use this link in your newsletters, social profiles/pages, or ask owners of other websites linking to your website to use it so that visitors coming from there see a specific Content Group.