When Microsoft SharePoint Foundation is installed, several Onet.xml files are installed—one in %ProgramFiles%\Common Files\Microsoft Shared\web server extensions\14\TEMPLATE\GLOBAL\XML that applies globally to the deployment, and several in different folders within %ProgramFiles%\Common Files\Microsoft Shared\web server extensions\14\TEMPLATE\SiteTemplates. Each file in the latter group corresponds to a site definition that is included with SharePoint Foundation. They include, for example, Blog sites, the Central Administration site, Meeting Workspace sites, and team SharePoint sites. Only the last two of these families contain more than one site definition configuration.
The global Onet.xml file defines list templates for hidden lists, list base types, a default definition configuration, and modules that apply globally to the deployment. Each Onet.xml file in a subdirectory of the %ProgramFiles%\Common Files\Microsoft Shared\web server extensions\14\TEMPLATE\SiteTemplates directory can define navigational areas, list templates, document templates, configurations, modules, components, and server email footers that are used in the site definition to which it corresponds.