This domain is used for DNS resolution as layer between the customers DNS and Fastly DNS so that we have a power to change the DNS resolution process if needed. 
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More on the custom domains here:
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<a href="https://kentico.atlassian.net/wiki/spaces/KTD/pages/3392634903/Custom+asset+domains">https://kentico.atlassian.net/wiki/spaces/KTD/pages/3392634903/Custom+asset+domains</a>
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<a href="https://kentico.atlassian.net/wiki/spaces/KTD/pages/3390505087/How+to+configure+custom+asset+domains">https://kentico.atlassian.net/wiki/spaces/KTD/pages/3390505087/How+to+configure+custom+asset+domains</a>