If your range hood or microwave recirculates back in to the room.

Both filters are required. The mesh filter catches the grease & air particulate. The carbon filter should be changed regularly www.HoodFilterFactory.com ( avoid range fires that spread quickly) & clears the recirculated air from smoke,odors, & airborne particulate.

With yours venting to the outside, you ONLY need the mesh filter. These filters should be cleaned regularly ( Avoid mentioned hazard) & should be replaced when you can no longer clean them enough to see through them when holding up to the light.

Good Luck