Difference between the synthetic magnitudes from the reference spectra and the corrected synthetic magnitudes from the XP spectra as a function of B_p−R_p colour for reference stars in the magnitude range that is relevant for SYMBIO-SIS standard stars (G < 10.5) for the SIMBIO-SYS STC filters. The pink triangles show SPSS stars, yellow squares show PVL stars, and grey circles show CALSPEC stars. The thick horizontal line marks Δmag = 0.0, and the thin horizontal lines enclose the Δmag = ±0.05 range.