  We study the BL Lac objects detected in the one year all sky survey of the
Fermi satellite, with a energy spectral slope alpha_gamma in the [0.1-100 GeV]
band greater than 1.2. In the alpha_gamma vs gamma-ray luminosity plane, these
BL Lacs occupy the region populated by Flat Spectrum Radio Quasars (FSRQs).
Studying the properties of their spectral energy distributions (SED) and of
their emitting lines, we find that several of these BL Lacs have a SED similar
to FSRQs and that they do have broad lines of large equivalent width, and
should be reclassified as FSRQs even adopting the current phenomenological
definition (i.e. equivalent width EW of the emitting line greater than 5 A). In
other cases, even if the EW width is small, the emitting lines can be as
luminous as in quasars, and again their SED is similar to the SED of FSRQs.
Sources classified as BL Lacs with a SED appearing as intermediate between BL
Lacs and FSRQs also have relatively weak broad emission lines and small EW, and
can be considered as transition sources. These properties are confirmed also by
model fitting, that allows to derive the relevant intrinsic jet parameters and
the jet power. This study leads us to propose a physical distinction between
the two classes of blazars, based on the luminosity of the broad line region
measured in Eddington units. The dividing line is of the order of L_BLR/L_Edd ~
5e-4, in good agreement with the idea that the presence of strong emitting
lines is related to a transition in the accretion regime, becoming radiatively
inefficient below a disk luminosity of the order of one per cent of the
Eddington one.
