PES Pareto is a method to categorize the strata of broadband noise in the frequency domain for a feedback (or feedforward) system.   The idea is to isolate noise measurements and then back filter their Power Spectral Densities (PSDs) through the appropriate magnitude squared filter to be able to model the noise as an independent noise input.  All of the various isolated independent noise inputs can then have their PSDs forward filtered through the appropriate magnitude squared filter to show their effect at any point in the loop.