The application has to be described as control loops with unique loop tags loops. This makes it possible, in documentation or discussion, to refer to a certain loop unambiguously, using the specified tag.

Each and every I/O belongs to a measurement or control loop. Each loop has to have a unique tag (sometimes referred to as tag code). A temperature control loop might be tagged as TIC-220 (T = temperature, I = indication, C = control, 220 = loop id number). An analog input for temperature measurement could be tagged as TI-220 or TE-220.1 (E = sensor, 1 = id identifying each of the several sensor inputs in this loop), an analog output controlling the power of heating device TIC-220.O (O = output) or TT-220.1 (T = signal transmitter).

Also, you have to define the type sensor or control device as well as the scaling - "flow measurement FI-202; current signalling 4..20 mA; scale 0..4,0 l/s".

Binary I/O is tagged and described accordingly: "Tank S-101 high limit LSH-101; closed when level < 95 %" or "Conveyor C-309 Speed monitor GS-309, inductive pulse sensor, normally open, 1 pulse/revolution, rotation speed 0..20 rpm, limit for skidding 18 rpm, stopping limit 2 rpm."