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Well with a carbon filter unless scrubbing (fan on filter no ducting,which recycles the air back into the grow) you're sucking exhaust air through it to clean the exhaust air of odors before dumping it outside or to another room.
HEPA filters suck the air through to trap unwanted mold/spores before fumping said air back.into the room (like in a central heat and air return).
Hepa filters remove particulates. Carbon filters remove organic gasses & some particulates, but the last part isn't the intended function. Choosing depends on what problem you're trying to correct.