The wet clutches in most automatic transmissions produce dust from the friction of the dozens of discs, much of which accumulates on the pan magnets. And I have always assumed that the difference in severe and normal duty service intervals on transmission is significantly determined by the level of wear and resulting metal dust generated. Transmission pan magnets are usually loaded with black iron from the plates and if the magnets become fully loaded eventually the filter will be plugged with the dust. And what happens then is expensive.