The set of architectures where no metric can be improved without making another one worse.
Instead of a single supposedly optimal plant, the Pareto front shows the best trade-offs between competing goals — return against robustness, self-sufficiency against CapEx. The dominant architecture is then chosen from it according to the stated objective.
A naive point optimiser returns one number; the Pareto front shows the room for manoeuvre and makes the choice of objective visible.