Root? I think you mean route.
In programmable logic controller vernacular, discrete devices are input or output devices that provide or receive discrete digital signals. A discrete digital signal is one that can report only two states, such as ON/OFF or open/closed. Binary.
A limit switch is an example of a discrete input device because, at any given time, it is either open or closed. It sends a discrete digital signal to a controller. This signal can have one of only two values, 0 or 1, indicating that the device is either OFF or ON, respectively.
I can do this all day.