Channel Purger vs Message Filter

It was not clear to me the difference between the 2:


http://eaipatterns.com/ChannelPurger.html this is meant to remove from a CHANNEL (IN PARALLEL TO THE MAIN FLOW) spurious bogus messages that should be prevented from reaching the main flow (e.g. test messages or old expired messages)

http://eaipatterns.com/Filter.html a filter is used IN SERIES TO THE MAIN FLOW to discard irrelevant (but perfectly legal) messages

So the difference is like between a parallel spike suppressor and a series filter (pardon me for being an electronic engineer).