Instrumentation and control cables carry the low-level signals that keep a plant observable — the readings from sensors and the commands to actuators. Because those signals are small, the cable's job is as much about protecting them from noise as carrying them.
Shielding and twisting do the heavy lifting. Twisted pairs cancel interference picked up along the run, while foil or braid shields drain it away to earth. The right combination depends on the environment: a quiet control room asks less of a cable than a floor full of variable-speed drives.
Get the construction right and the payoff is clean data and stable control; get it wrong and you chase intermittent faults for months. That's why instrumentation cable is specified by its screen, its conductor and its standard — not by the metre.