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>