[EN 12792:2003, 151]:air terminal unit of the following types:constant flow rate type (also referred to as series type)[EN 12792:2003, 151]:assembly within which the primary air flow rate is modulated and mixed withair induced from the surrounding atmosphere, secondary air, by means of a continuously operating integral fan in order to provide a relatively constant flow rate of air to the treated space.variable flow rate type (also referred to as parallel type)[EN 12792:2003, 151]:assembly within which the primary air flow rate is modulated and mixed with air induced from the surrounding atmosphere, secondary air, by means of a non-continuously running fan, but which is operated in order to provide a variable flow rate to the treated space in response to thermal loads.