Metallicity distribution of calibrating clusters: the top panel shows GCs and the bottom panel OCs. Heavily shaded histograms show clusters included in iDR4; lightly shaded histograms clusters that will be included in future releases (see also Tables 8 and 7); empty histograms represent a pool of viable candidates to complete the coverage. A few of them might be selected in the next observing runs, depending on observations scheduling and data homogenization needs.