The relativistic beaming model is adopted to discuss quantitatively the obs
ervational differences between radio-selected BL Lac objects (RBLs) and X-r
ay-selected BL Lac objects (XBLs), and between BL Lac objects and flat spec
trum radio quasars (FSRQs). The main results are the following: (1) In the
Doppler corrected color-color (alpha (in)(ru) - alpha (in)(ox)) diagram, XB
Ls and FSRQs occupy separated regions, while RBLs bridge the gap between th
em. These properties suggest that similar intrinsic physical processes oper
ate in all the objects under a range of intrinsic physical conditions. (2)
Our results are consistent with the results of Sambruna, Maraschi and Urry
(1996) from other methods. We show the alpha (xox) introduced by Sambruna t
o be a good index for describing the energy distribution because it represe
nts the intrinsic energy distribution and includes the Doppler correction.
(3) The Doppler effect of relativistic beaming is the main mechanism, and t
he physical differences (such as magnetic fields, electron energies) are al
so important complementary factors for understanding the relation between X
BLs and RBLs.