Two-dimensional parameter space of log Z − log U illustrating the physical conditions of the gas surrounding xA sources with different SEDs. In each plot, the minimum χ² computed between measured diagnostic ratios and CLOUDY simulations is shown with dots: green for the Mathews & Ferland (1987) SED and blue for the xA SED. The orange region corresponds to the 1σ accuracy range for the Mathews & Ferland (1987) SED, and the blue region for the xA SED.