The Synchrotron theory predicts polarized emission whose characteristics depend on the optical depth of the emitting material and the magnetic field uniformity. The transition between optically thick and thin states, uniquely observed in radio bands, is expected to be correlated with the measured polarization degree, and is associated with polarization angle rotations of 90° and reversal of the Circular polarization handedness (Pacholczyk 1977). Furthermore, theoretical predictions for the radiative transfer coefficients suggest a connection between the Linear and Circular polarization characteristics and the emitting plasma state (e.g., cold or ultra-relativistic, Huang et al. 2011) and composition (i.e., e-/e+ or e-/p+ jet, Wardle et al. 1998). The recent discovery of optical polarization angle rotations associated with high-energy flares, on the other hand, provides an independent and complementary approach to the problem (Marscher et al. 2008).