Exactly. You want to vent the fumes to the outside, and also trap the solids (the paint overspray) in the filter. It's important that the vapors are vented OUT of the paint room and to the outdoors, not just run through a paper filter and recirculated into the same room, because the filter does not trap the toxic fumes. That's why the vent must be open to the outdoors. Theoretically you don't even need the filter; as long as you're venting the fumes to the outside, you're ok. But unfiltered, the overspray will tend to settle around the vent opening and make a bit of a mess, so having a paper filter in the vented airflow is a good idea.