CombinedNavSiteMapProvider was initially introduced in SharePoint Server 2007, which included a master page named BlueTabs.master. The BlueTabs.master page used the
CombinedNavSiteMapProvider by default, which also provided an additional tier of navigation in the top-level navigation (see Figure 17.119.)
By using the same concept as in BlueTabs.master, you can adapt the
CombinedNavSiteMapProvider to add two tiers of menu items into SharePoint Server 2010 sites.