Product review (5 stars): For vents that blow outside.
Man, I was really confused about the whole exhaust versus duct versus ductless thing, but lots of research led me to these & maybe I can help others. Apparently, the previous residents were using the wrong type of filter even though they're the same size & both fit.

If you turn your hood fan on & it blows directly outside through a hole/duct in your wall, then this is the filter you want. It is just a metal mesh, called a grease filter & I guess just captures grease splatters from the air that is being sucked & exhausted out to the outside...
If you turn your fan on & it sucks up air but then blows it back out directly above the hood and/or around cabinets, then there's no duct to the outside. This set-up is common when the microwave is installed above the stove. Because the air is being recycled back into your living area, you need to purchase the "non-duct" version which is exactly the same size but it also has a black/blue charcoal screen inbetween the mesh, which will filter & trap any smells from blowing back into your air.