Can anyone tell me what's the logic of VAV - zoning system & operation?

VAV ( variable air volume) system is to control the rooms a/c by varying the air flow to the served area by means of a vav terminal boxes. The air andling unit shall be vav as well.
The main function of a vav system is to provide individul control over each zone served by a vav terminal box though they are served by only one air handling unit. The main application for it is in office areas for example where each office needs to control the a/c (air flow) individually with minimum effect to the other zones.