Site servers automatically serve the site to which their parent subnet is assigned. Site servers may have multiple NICs/IPs and be in more than one subnet, so may therefore belong to more than one site. You can also manually assign each site server to one or more other sites. The Manually Assigned column in the Detailed Information table indicates whether the site server is manually assigned to the site.
See DOC7978 on page 113.
When you manually assign a site server to a site, only the site server is assigned to the selected site. The subnet to which the site server belongs is not affected.
To assign a site server to a site manually