There is many types of balancing control valve.
You might want to get someone in the field to explain you about this. Suppliers / AC contractor/ Engineers, etc.
But simple answer for your simple question,
What is important for AHU / FCU is the chilled water flowrate required for the cooling.
So one way to control it by control the differential pressure to give correct flowrate required. CHW flowrate will modulate according to the heat load required in the A/C space.
To install at the return side of the CHW pipe (leaving CHW pipe, after the AHU / FCU).
Function - to control water flow rate to either a fixed flow rate or variable depending on cooling load required ( probably controlled on exit temperature)
Position? Upstream or downstream of the unit. downstream means you won't run into air problems, but upstream could reduce pressure if this is what is required.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
As others have stated, your question is too vague to answer the proper application. A delta P valve is intended to replace a control valve and a balance valve in a 2 way application. One must review the application from a system level not just a single terminal. These valves can be used on heating or cooling coils but one need the primary pumping design information for a proper selection. These will work with variable pumping designs. Most commonly found on leaving side of coil. I would use caution on an application of limited retrofit due to unknown system flow rate and coil dp on other terminals.
