As we already know, a current value of 4mA represents 0% of the scale, a current value of 20mA represents 100% of the scale and any current value between 4 and 20mA represents a corresponding percentage between 0% and 100%.
This is because it is easier for us to detect if there is an open circuit (broken wire/fault) on the instrument itself.
We use 4mA as a “live zero”. That means if we measure the current at OmA, the instrument is probably faulty or our circuit is in an open loop.