A register is a grille with moving parts, capable of being opened and closed and the air flow directed, which is part of a building's heating, ventilation, and air conditioning (HVAC) system. The placement and size of registers is critical to HVAC efficiency.
A diffuser is "a device for reducing the velocity and increasing the static pressure of a fluid passing through a system”. Diffusers are used to slow the fluid's velocity while increasing its static pressure. The fluid's static pressure rise as it passes though a duct is commonly referred to as pressure recovery.
A zone damper (also known as a Volume Control Damper or VCD) is a specific type of damper used to control the flow of air in an HVAC heating or cooling system. In order to improve efficiency and occupant comfort, HVAC systems are commonly divided up into multiple zones.