Point-sources flux limit (in the 0.5–2 keV energy range) vs. area scatter plot of a few selected X-ray surveys larger than 1 deg². Existing surveys from Einstein (light green downward triangle), ROSAT (dark green upward triangles), XMM-Newton (blue circles), and Chandra (purple squares) are shown for reference. Filled points mark contiguous surveys, and empty points show non-contiguous ones. To allow a fair comparison, we considered the total area that was covered (x-axis) for each survey and a flux limit that corresponds roughly to a completeness level of 66%. The dotted lines mark the loci of constant source numbers based on the number counts in the 0.5–2 keV energy rage of Mateos et al. (2008) (double power-law model). The surveys from left to right are The Subaru/ XMM-Newton Deep Survey (SXDS; Ueda et al. 2008); the XMM-COSMOS survey (Cappelluti et al. 2009); the COSMOS-Legacy survey (Civano et al. 2016); the XMM-Newton Medium Survey (XMS; Barcons et al. 2003); the XMM-Newton Bright Survey (XMM-BCS; Della Ceca et al. 2004); the XMM-RM survey (Liu et al. 2020); the XMM-ATLAS survey (Ranalli et al. 2015); the Chandra Deep Wide Field Survey (CDWFS; Masini et al. 2020); the Chandra Multiwavelength Project (ChamP; Kim et al. 2007); XMM-SERVS (Brandt 2020); the ROSAT International X-ray/Optical Survey (RIXOS; Mason et al. 2000); XMM-XXL N & S (Chiappetti et al. 2018); Stripe82X (LaMassa et al. 2016); NEP (Henry et al. 2006); the second Chandra Serendipitous Sources catalogue (CSC2.0; Civano, priv. comm.); the Einstein Observatory Extended Medium-Sensitivity Survey (EMSS; Gioia et al. 1990); the fourth XMM-Newton serendipitous source catalogue (4XMM; Webb et al. 2020), and the second ROSAT all-sky survey catalogue (2RXS Boller et al. 2016).