If the Brand URL Filter in one of your themes exactly matches a segment of your domain name, users who log on using that domain name will see the associated theme. A segment is the portion of a domain name that is between periods (.).
Example
You have purchased the domain name abc.graphicdesign.com, and that domain name resolves to your server. You create a theme with a Brand URL Filter of abc. When a user logs on to abc.graphicdesign.com, they will access your server, and they will see the theme you created for ABC Graphic Design.