Sky coverage of the XXL-100-GC sample, displayed as a function of the total XMM count rate (i.e. including the three imaging cameras and integrated to infinite radius). Left: total sky coverage for different cluster core radii. These curves are the slices through the two-parameter selection function displayed in Fig. 9. Right: ratio of sky coverage between the northern and southern fields. For sufficiently large count-rates, the sky coverage of the northern field is consistently 10–15% larger than in the south, while the difference is more pronounced when the total sky coverage falls below 1 deg².