[N ii]-BPT ([O iii]λ5007/Hβ versus [N ii]λ6583/Hα) diagnostic diagram for the SMBHB candidates. The black curve marks the separation proposed by Kewley et al. (2001, denoted as Ke01 in the diagrams), between the theoretical maximum ionization driven by pure star-formation in HII regions and those regions ionized by AGN. The red solid line on the BPT diagram marks the separation between AGN and Low Ionization Narrow Emission-line Regions (LINERs), as proposed by Schawinski et al. (2007), while the blue solid curve represents the separation between AGN and star formation reported by Kauffmann et al. (2003) denoted as Ka03 on the [NII]-BPT diagram. The markers are color-coded according to the observed eROSITA luminosity of the bright level in the 0.2–2.3 keV band. The numbers of the markers correspond to the sources listed in Table 1.