A zone, as it relates to HVAC systems, is an area composed of a portion of a building, a whole building, or several buildings that are serviced by an air handling unit (AHU). There is typically one AHU per zone. In some cases, multiple AHU's are utilized to create inter-zones within a larger zone. Sensing units can be used to monitor the zone for occupancy which in turn determines where in the zone airflow is needed. For smaller commercial and residential buildings, smart dampening or mini split systems can be used. These systems are able to utilize a single AHU to manage the air condition of multiple zones.

Image source: www.acreeair.com