Performance and standardisation of JKC U band XP synthetic magnitudes using the reference sample described in Sect. 3.2. Left set of panels: Δmag as a function of G magnitude for the entire sample using nominal XP synthetic magnitudes (left panel) and standardised XP synthetic magnitudes (right panel). In each panel, the continuous red line connects the median Δmag computed in 0.2 mag wide bins, 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 panel of each panel. Right set of panels: the same for Δmag as a function of BP-RP colour, limited to the subsample of reference stars having XP spectra released in DR3.