Sketch illustrating definition of STs along MS, as function of R_FeII and FWHM(Hβ). The numbers in square brackets yield from top to bottom, the prevalences of each spectral bin (n_ST) in an optically selected sample (Marziani et al. 2013), of the RI and RL sources (n_RI + RL) and of RD (n_RD) in each spectral bin. The total number of sources is N = 680. The gray and pale green ares trace the source occupation in the plane; the green area separates extreme Population A from the rest of the MS.