Variable inputs are inputs which enable the user to vary the input from any particular source. Typically these are most commonly used as an active speed control with 4-20mA or 0-5V inputs being perhaps the two most common. These inputs offer a simple method by which the user or PLC can directly control the input and vary it according to the application requirements.
However, there are a number of more advanced configurations which can be setup with two variable inputs which can enable the controller to fulfill a much broader range of functions.