A.P.O. stands for "Pull Out". The drains noted as being a P.O. Plug will also have overflow holes as these drains are designed to be used in sinks with overflow holes. C.O. stands for "Closed overflow". These drains will also sometimes come with a pull out plug, but more importantly the C.O. style drains do not have overflow holes. C.O. style drains should only be used in sinks that do not have overflow holes.