0.5–2 keV (left) and 2–10 keV (right) area-flux curves for the AXIS deep, 5 Ms (solid black line), intermediate, 300 ks (dashed black line) and wide, 15 ks (dotted black line) reference surveys. For comparison, we show the area-flux curves of several existing X-ray surveys: CDF-S 7 Ms (Luo et al. 2017, dotted blue line); CDF-N 2 Ms (Xue et al. 2016, dashed gray line); AEGIS XD (Nandra et al. 2015, dotted purple line); SSA22 (Lehmer et al. 2009, dotted light green line); XDEEP-2 F1 (Goulding et al. 2012, dashed yellow line); J1030 (dashed pink line; Nanni et al. 2020); X-UDS (Kocevski et al. 2018, dashed brown line); Chandra COSMOS Legacy (Civano et al. 2016, dashed cyan line); X-Bootes (Murray et al. 2005, dashed magenta line); Stripe 82X (LaMassa et al. 2013b,a, 2016, dashed green line); and XMM-XXL (Pierre et al. 2016, dashed orange line). We also show the predictions made for the ATHENA deep– and wide–area survey (dashed red lines; see the text for more details), and those for four years of eROSITA observations (i.e., eRASS:8; Merloni et al. 2012; Comparat et al. 2019, dashed light green line). The plotted lines have been derived from the 0.5–2 keV survey sensitivity curves in an area range that starts at 20% and stops at 80% of the area covered by the survey.