  We discuss the features that mark the Flat Spectrum Radio Quasars from the BL
Lacertae objects. We propose that FSRQs exceeding L = 10^{46} erg/s are powered
by black hole accreting at rates dm/dt = 1 - 10; their power is dominated by
the disk components, thermal and BZ. Instead, sources accreting at rates
10^{-3} < dm/dt < 10^{-2} radiate in the BL Lac mode; here the power is mainly
non-thermal, and is driven by the rotational energy stored in a Kerr hole which
sustains L < 10^{46} erg/s in the jet frame for Gyrs. The two populations may
be even linked if around the same objects dm/dt drops in time; then some
negative cosmological evolution is expected for the newborn BL Lacs fed by the
dying FSRQs. Further implications are discussed.
