Exact definition & difference of FCV and ON / OFF vavles

The difference is in terms of design requirements.

The FCV is used where there is continuous control of the liquid. It opens and closes depending on the quantity of liquid to be drained to mainain a particular constant level. That means the opening and closing varies depending liquid to be flowed. Example is a liquid in a live vessel that is continuously being controlled depending on the liquid level in the vessel. Most times, the FCV is used to maintain a particular liquid level in the vessel.

For the On/OFF valves, It is like a digital device. It comes On and opens the valve 100% and goes off and shut the valve. The valve does not respond to varying amount of liquid to be controlled. That is it either opens 100% or closess 0%. It is used in vent scrubbers, sump tanks or knockout vessels

The FCV can open 20%, 30%, 40% to 100% depending of the quantity required to be flowed.

Yes the FCV is used to control the flow thereby altering the level in the downstream or upstream of the valve.Whereas On-off valve is always called upon to perform only one service either to be on or off no intermediate position is contemplated.

depending on the two extreme signals the On-off valve is made to fuction.Whereas the Control valve is made to fuction for a range.

As regards your standards though I'm not immediately be able to answer but if you can refer ISA standards you should be able to find a clue.