Optimal operation usually refers to the nominally optimal way of operating a plant as it would result by applying steady-state and/or dynamic optimization to a model of the plant
(with no uncertainty), attempting to minimize the cost by adjusting the degrees of freedom.
In practice, we cannot obtain optimal operation due to uncertainty. The difference between
the actual value of the objective function and its nominally optimal value is the loss.
The two main sources of uncertainty are (1) signal uncertainty (includes disturbances (!d ) and measurement noise ( n")) and (2) model uncertainty.
Robust means insensitive to uncertainty. Robust optimal operation is the optimal way of operating a plant (with uncertainty considerations included).