Origin of the two distinct solutions in Fig. B.2 is highlighted in these two cartoons. Given the data shown in the red thick line, one can either A) consider a bright, highly obscured source (top panel), or B) a low-luminosity, low-obscuration solution where the hard counts are due to the background. An intermediate solution is ruled out however. Depending on the background level and redshift, the two solutions will have different likelihoods.