Is it an Under Gravel Filter or Plenum; or is it a Dirt Magnet or Bio Sponge Filter? 

Is it an Under Gravel Filter or Plenum; or is it a Dirt Magnet or Bio Sponge Filter?
A couple of comments have come in that all I'm talking about is under gravel filters and that's what I should be calling it: an under gravel filter. However, if an under gravel filter has a void underneath it then that's a Plenum. Does it change if you pump water through the Plenum slowly or do it as Jaubert says to do it?. Does that now change one to a Plenum and one to an under gravel filter because they both have a void underneath them? Or would you call an under gravel filter something that only uses half inch piping and the substrate is laid right on top of it because then there is no Plenum.