Monochromatic 2 keV flux densities obtained from XMM-Newton versus eROSITA data. The solid black line indicates the bisectrix. The best fit is shown by the dashed line, and 3σ uncertainties are indicated by the shaded area. Red circles and blue squares indicate whether eROSITA data come from eFEDS or eRASS1, respectively. The legend also reports the values obtained for the slope (ζ) and intercept (ξ).