A tab panel could contain anything including another tab boxes.

<tabbox>
    <tabs>    
        <tab label="First"/>        
        <tab label="Second"/>        
    </tabs>    
    <tabpanels>    
        <tabpanel>        
        The first panel.        
        <tabbox>        
            <tabs>            
                <tab label="Nested 1"/>                
                <tab label="Nested 2"/>                
                <tab label="Nested 3"/>                
            </tabs>            
            <tabpanels>            
                <tabpanel>The first nested panel</tabpanel>                
                <tabpanel>The second nested panel</tabpanel>                
                <tabpanel>The third nested panel</tabpanel>                
            </tabpanels>            
        </tabbox>        
        </tabpanel>        
        <tabpanel>The second panel</tabpanel>        
    </tabpanels>    
</tabbox>