The SDS identifies and documents the characteristics of the resources that are available to
estimate values of the monitored quantities and to set values of the controlled quantities. These
values are usually read from or written to hardware devices, such as sensors and actuators. The
values in the system's hardwarehoftware interfaces are denoted by variables. These variables are
partitioned into input variables, values read by input devices, and output variables, values written
to output devices. The product of this step is a description of the input and output devices and of
the relationship between the input and output variables and the monitored and controlled
variables.