Distribution of magnitudes and formal positional uncertainties for the sources common to Gaia-CRF3 and ICRF3 S/X. The full set of 3142 sources is shown by the grey histograms, and the 259 defining sources by the red histograms. Left: distribution of magnitudes G. Middle: distribution of positional uncertainties in Gaia EDR3. There are 126 sources with mas, of which eight are defining sources in ICRF3. Right: distribution of positional uncertainties in ICRF3 S/X. There are 155 sources with mas, of which one is a defining source.