Good question but I believe there are a few -
CRANKCASE VENT FILTER-- Prevents dirt from entering crankcase.
AIR FILTER-- Keeps abrasive dust, dirt, bugs and other foreign matter out of the engine air intake.
ENGINE OIL FILTER -- Traps dirt, metal filings and sludge, allowing only filtered oil to circulate through the engine.
TRANSMISSION FILTER-- Collects dirt and metal filings, protecting finely machined parts.
FUEL FILTER-- Traps rust, sediment, dirt and other particles which may enter the fuel system and clog tiny jets.
VAPOR CANISTER-- Collects gasoline vapors and returns them to the fuel system.