I'll stick up for NCPE here; He is correct that circuit setters do not need to be installed at every terminal (read coil), you also can use venturi style balancing valves. They're a little more expensive, but can be separated from the circuit setter title.
How many people understand the effect of fluid dynamics? Path of least resistance? Lets say that the system has 440 cfm at 100 ft. head. What would happen to a coil that is about 50 feet (pipe length, size doesn't matter) from the pump, and needs 15 cfm to heat or cool properly (there are going to be a buttload - engineering term - of terminals in the system). Without a circuit setter (or balancing valve), a lot more than 15 cfm is going to pass through the terminal. This is why it's called balancing the system. You "balance" the flow to supply the required flow to each and every "terminal" in the system.
In essence a circuit setter is a bottle neck. It creates pressure loss to allow the proper conditions to be met in the terminal, to allow proper use of the unit.
*Lots of explanations because some people can't read between the lines, and need it spelled out for them. And buttload was used because that's just fun.
