If my Input is above Setpoint, should the output be increased or decreased? Depending on what the PID is connected to, either could be true. With a car, the output should be decreased to bring the speed down. For a refrigerator, the opposite is true. The output (cooling) needs to be increased to bring my temperature down.
This function specifies which type of process the PID is connected to. This information is also specified when the PID constructed. Since it's unlikely that the process will switch from direct to reverse, it's equally unlikely that anyone will actually use this function.
Direction: DIRECT (like a car) or REVERSE (like a refrigerator)