I'm a bit confused by the words "box filter", "HOB filter", "corner filter", and so on. They aren't very self-evident, since I'm not sure which one refers to an air-run sponge filter, a power filter, and so on. I know HOB means "hang on back", but does it mean an internal or external filter? And what are my filters called, when they're both internal filters driven by engines and have sponge as filter material? I'd be very grateful if someone more familiar with the English terms could explain me this .