The first identification letter is for the measured value, the second is a modifier, 3rd indicates the passive/readout function, 4th - active/output function, and the 5th is the function modifier. This is followed by loop number, which is unique to that loop. For instance, FIC045 means it is the Flow Indicating Controller in control loop 045. This is also known as the "tag" identifier of the field device, which is normally given to the location and function of the instrument. The same loop may have FT045 - which is the flow transmitter in the same loop. For reference designation of any equipment in industrial systems the standard IEC 61346 (''Industrial systems, installations and equipment and industrial products — Structuring principles and reference References Control engineering Control loop theory