I think RootWebOnly attribute above can be explained a little bit further. If you like to create a site definition that is only used to create top level sites , in the Configuration element of WebTemp*.xml , you specify RootWebOnly = "True ". This ensures that your site definition is only used for creating top-level sites which also means it's only available in Central Administration site where you create the site collections.
Reza Alirezaei, MVP
Blog: http://blogs.devhorizon.com/reza