If your BBQ is built in to cabinetry affixed to an external wall then you need a wall mounted BBQ Rangehood. Howver should you have an opening in that external wall such as a window you may not have enough wall surface to attach a BBQ Rangehood. In this instance you will need a ceiling mounted Rangehood.Other instances will be when there is no external wall available.
Wall Mounted Rangehoods are are fixed to the external wall and are open on three sides. The amount of exhaust required is determined by the depth of the Hood & where the motors are placed.