HVAC stands for heating, ventilation, and air conditioning. In computing and especially in enterprise data centers, HVAC systems control the ambient environment (temperature, humidity, air flow, and air filtering) and must be planned for and operated along with other data center components such as computing hardware, cabling, data storage, fire protection, physical security systems and power. The selection of an HVAC contractor is an important step in planning a data center. Almost all physical hardware devices come with environmental requirements that include acceptable temperature and humidity ranges. Environmental requirements are usually described in a product specifications document or in a physical planning guide. A separate space, called a plenum, is often allocated to house and allow air circulation for HVAC and communication cabling, typically in the space between the structural ceiling and a drop-down ceiling or under a raised floor.