Sky coverage of the XXL-100-GC sample, displayed as contours of constant sky coverage in the source parameter space – cluster core radius for a β-model with β = 2 / 3 vs. total XMM count rate, summed over the three imaging cameras and integrated to infinite radius. For comparison, the dashed red curve shows the aperture flux cut of CR₆₀ = 0.0332 cts/s used for the subselection of the bright XXL cluster sample.