What is the need for test terminal which is near to the transmitter supply terminal

What is the purpose of Test terminal available near by transmitter supply terminal and how it works :- Mainly for Loop powered transmitters.

it is there so that you can measure current without breaking the loop. It is a diode forward biased, the low impedance of the ammeter shorts the diode and prevents it from getting its voltage drop which shuts it off and passes all loop current through your meter. When you disconnect you get the voltage drop to turn the diode back on. It doesn’t interrupt the loop to do this