Effect of stacking RMFs. Left panel: Observer-frame eROSITA RMF of a single source. Here the color indicates the probability. Weak off-diagonals are visible. Middle panel: Rest-frame “stacked RMF” extracted from the stacked full response of the spec-z sample. Off-diagonals are less prominent, while the width of the diagonal becomes larger compared with a single RMF. Right panel: Spectral energy dispersion (FWHM of the RMF matrix diagonal) at different input model energies (observer-frame for the single RMF, while rest-frame for the stacked RMF) quantifying the spectral resolution. The energy dispersion increases with energy (resolution decreases with energy). The observer-frame energy dispersion of a single source (red curve) is lower than the rest-frame energy dispersion of the spec-z sample (blue). The standard measurement of eROSITA energy resolution from Table 4 of Predehl et al. 2021 (7 TMs averaged) is also listed as orange diamonds.