it represents a situation where the measure process value like temperature or pressure from the sensor is below the specified range or current value is below the 4mA of reading.
Pressure sensors giving a voltage output are essentially based upon the mV
output transducers. Additionally they have further electronics fitted internally
within the sensor to amplify the mV signal from the Wheatstone bridge
Multiple voltage readings taken at a specific sampling rate.
Usually the FF transmitter has the transducer calibrated for factory set range like in case of thermocouple, it may be -250 to 1296 degC say. But the process required it to measure under a value of -100 to 100 deg c to which the transmitter will be linearised and calibrated. this is called the XD_Scale.
In certain cases like a DP transmitter which can be used to measure level then the XD scale will be range in terms of pressure whereas the Out_Scale of the transmitter to the DCS will be range in terms of % of level.
Type of shunt-diode barrier that uses a high-quality safety ground
connection to bypass excess energy
Voltage Output Transducers