Performance and standardisation of SDSS ugriz XP synthetic magnitudes using the selected subset of the Thanjavur et al. (2021) sample, which we adopted as reference. Left set of panels: Δmag as a function of G magnitude for the entire sample using nominal XP synthetic magnitudes (left panels) and standardised XP synthetic magnitudes (right panels). In each panel, the continuous red line connects the median Δmag computed in 0.2 mag wide bins, and the dashed red lines connect the loci of the 15.87% (P₁₆) and the 84.13% (P₈₄) percentile computed in the same bins. The median (P₅₀) and the difference between P₈₄ and P₁₆ – here used as a proxy for the standard deviation σ – for the entire sample are reported in the upper left corner of each panel. Right set of panels: Same for Δmag as a function of G_BP − G_RP colour, limited to the subsample of reference stars with XP spectra released in Gaia DR3.