A “regional” is what you remove from the data to better image what you want to “see” (residual). The best regional/residual combination is achieved by applying geologic thought to the problem. What is the geologic target? The hypothetical curve represented below illustrates the residualization concept, the idea of zeroing in on a particular anomaly or wavelength, simplifying the profile and, hopefully, the interpretation. If your goal is a better understanding of the asthenosphere, you may want to capture the very long wavelength blue anomaly. The red anomaly may shed light on crustal thinning, while the green anomaly relates to basin depth. The short-wavelength orange anomaly may be caused by the hydrocarbon trapping structure.