Distribution of the SOU/BKG indicator for MOS2 as a function of scaled radius, for all the 354 regions from which we extracted spectra. The color coding refers to the extension in the sky (R₅₀₀ in arcmin) of the corresponding cluster. The horizontal continuous line shows the value where the source intensity gets lower than the background line, while the dashed line marks the threshold SOU/BKG = 0.2 used in the analysis of mean and median profiles (Sect. 6.2). The vertical line marks R₅₀₀. Most of the regions below the threshold SOU/BKG = 0.2 at R < R₅₀₀ belong to very extended clusters and are thus located in poorly exposed regions of the FOV.