Distribution in the M₅₀₀ − z plane of confirmed clusters from major SZE surveys, available at the time of proposal submission (October 2017). Filled circles: Planck clusters (Planck Collaboration VIII 2011; Planck Collaboration XXIX 2014; Planck Collaboration XXVII 2016); triangles: SPT (Bleem et al. 2015); squares: ACT (Hasselfield et al. 2013). Masses for Planck clusters are derived iteratively from the Y_SZ–M₅₀₀ relation calibrated using masses from XMM-Newton; these were not corrected for any HE bias (see text for details). The figure includes both masses published in the Planck catalogue, and new masses computed using new redshift information. The shaded boxes indicate the Tier-1 and Tier-2 redshift ranges in blue and orange, respectively. The sample is drawn from the Planck PSZ2 sample, selecting clusters detected at high signal-to-noise (S/N > 6.5) with the MMF3 algorithm, and in the cleanest part of the sky. We also excluded clusters in the sky region with poor XMM-Newton visibility. Additional redshift, sky area, mass criteria, are applied to define the Tier-1 (0.05 < z < 0.2;  Dec > 0) and Tier-2 (z < 0.6, M₅₀₀ > 7.25 × 10¹⁴ M_⊙) samples. Remaining clusters in the shaded part of the M₅₀₀ − z plane are at lower S/N, or lie outside the sky regions under consideration. A full description of the sample strategy is given in Sec. 3.1 and is further illustrated in Appendix A.