Fractional variation of hardness ratio as a function of N_HX for fixed values of the covering factor, C_F. Blue labels specify the value of C_F for which each group of curves is obtained. For each C_F value chosen, solid black curves represent the results of Suzaku-XIS simulations for type-1 sources, whereas the long-dashed red ones show those derived for type-1 with XMM-Newton-EPIC/PN. When present, the third short-dashed red line of each C_F-value group of curves represents the case for type-2 XMM simulations.