Some devices being controlled by a dimmer should only be turned on and off; gradations of dimming are inappropriate.  Examples of these kinds are load are relays, solenoids, and pneumatic valves.  In cases like this, a non-dim curve is used.  The RC4 non-dim curve uses hysteresis to ensure the output will never unintentionally oscillate, even if the controlling DMX channel rises or falls very slowly or lingers near the center point.