Sorry, not a good enough explanation...I have seen all three filters inside the tank. The filter disc is extremely fine, the sock is fine, and the pump discharge "injector style" filter is of xx micron (don't know the filtration specifics, but it is VERY EXTREMELY FINE ).
Very fine yes, but nevertheless it is still just a screen. A very fine screen. But the paper external filter gets the really fine stuff. One thing we mechanics many times do, when we are curious (as you are), is we cut open filters to see what it was that had clogged them. Don't change your filter next time until it starts running like it's clogging up, and then cut it open and look inside. You see all the really fine mush (almost looks like really fine, fine wet beach sand or else that metal/ clutch wear slime that you find built up on the internal magnet on the inside of an auto tranny pan). Just make sure that if you cut it open, your cutting method wont cause any sparks. I always just use some tin snips or something, but don't use a hacksaw, as the friction can generate sparks and that would not be good for your eyebrows
).