VAV means Variable Air Volume
This is a device used on the ducts to change air flow as per the requirements. It has its own thermostate. It is used when we have different cooling zones.
This can be used with a VSD AHU. For example if we have a meeting room connected with ducted AHU and sometimes we need cooling in that room and some times not. When we put thermostates on cooling and the room temp is over that unit then VAV will open and cool air will start coming in that room and when the required cooling temp is achieved then the VAV will close slowly and completely.
Hope this will work