a)–c) The spectral fitting residuals reported in BU14 (top right panel of their Fig. 5), BO14 (right panel of their Fig. 6), and U15 (left panel of their Fig. 2), together with the expected residuals, shown as red curves, from our scenario described in Sect. 3.2. Emission measure of the CX model in c) is half of those in a) and b). As described in Sect. 3.2, the data-to-model deviation at ~3.9 keV in c) might be caused by other plasma emission lines. d) Fitting residual from a 1 Ms simulation of the XMM-Newton PN observation of the Perseus cluster. Details are described in Sect. 3.3. Vertical lines indicate the typical blended CIE and CX lines and the Fe xxv transitions in the 6.6–7.9 keV.