Comparison of X-ray to optical flux for several discussed samples: 55 X-ray detected (large red dots and arrows) and 22 X-ray undetected (large green arrows) sources in the RF5 AGN sample; 225 sAGN1 in the LS (blue squares and arrows); entire X-ray sample (i.e., all the VST sources with X-ray counterparts, gray dots and arrows), shown as a reference. Small leftward arrows denote VST-COSMOS sources which are detected in the Chandra-COSMOS Legacy Catalog but only have 2−10 keV flux upper limits, while large green arrows denote the 22 RF5 AGN candidates which remain undetected in any X-ray band. For the 22 X-ray undetected sources, hard-band X-ray flux limits are estimated using CSTACK (Miyaji et al. 2008). The dashed line indicates X/O = 0, while the lower and upper solid lines correspond to X/O = −1 and X/O = 1, respectively, delimiting the traditional AGN locus. The vertical dotted line indicates the nominal hard-band flux limit of 2.1 × 10⁻¹⁵ erg cm⁻² s⁻¹ of the Chandra-COSMOS Legacy Catalog, estimated at 20% of the area of the whole survey. The gray-shaded rectangle indicates that we do not find sources in that area as we set a magnitude limit of 23.5 mag for the VST-COSMOS dataset.