Intensity (upper left panel), V/I (upper right panel), Q/I (lower left panel), and U/I (lower right panel) profiles of the Mg IIh & k lines as a function of wavelength. The various colored curves show the results of reference PRD calculations in which the impact of the magnetic field is fully taken into account in all RT coefficients (black curves), neglected only in the computation of the emission coefficient (red curves), and only taken into account in η_I and ρ_V (blue curves). The magnetic field is horizontal, with χ_B = 0° and a strength of 20 G.