Sitecore supports the management of multiple website in one CMS environment. All sites could have a different domain name, and the same Sitecore backend. This is very convenient, because it increases the reuse of controls for developers, and the reuse of content for content managers.
But because all sites have a unique domain name, they should also have a unique Sitemap.xml file for Search Engines. The Sitemap.xml file is submitted to Search Engine Robots to index the pages of the the site.