Identification fraction of sources listed in the 3PBC computed as the ratio of the number of unidentified sources (N_unid) to the total number of sources (N_src) with flux in the hard X-ray band 15–150 keV, above a certain threshold. The blue and red lines indicate the hard X-ray flux for which the 3PBC has 90 and 95% identification, respectively. About 85% of the sources in the 3PBC appear to have an optical spectroscopic classification.