Left panel: difference in the metallicity estimates from the AlIII/HeIIλ1640 intensity ratio and from the CIV/HeIIλ1640 ratio of the broad component, as a function of ionization parameter and density. The difference remain significant at a 2σ confidence level down to Δlog Z = log Z(AlIII/HeIIλ1640) − log Z(CIV/HeIIλ1640) ≈0.3. Right: same, for difference in the metallicity estimates from the (SiIV + OIV])/HeIIλ1640 ratio (≈SiIV/HeIIλ1640) and from the C IV/HeIIλ1640 ratio. The two cases with red values are lower limits.