Extraction is the way that the filtered air travels through the hood.
Ducted hoods pass the air outside via a pipe to completely remove the smells and grease from the kitchen.
Recirculatory hoods draw the air through a filter to trap the odours and grease before passing clean air back into the kitchen.
The extraction rate is the speed at which the air is removed and is measured in m3 per hour.
It is generally recommended that a hood be able to process 12 times the volume of the room per hour so the larger the room the more powerful the motor and the fan in the hood need to be. More powerful motors are usually louder so are not recommended for smaller kitchens.