<Tr> <Td> author </Td> <Td> author * </Td> </Tr> <Tr> <Td> category </Td> <Td> category </Td> </Tr> <Tr> <Td> channel </Td> <Td> feed </Td> </Tr> <Tr> <Td> copyright </Td> <Td> rights </Td> </Tr>

Itunes and google reader are both examples of a content aggregator