Decomposition of the C IV absorption structure at a particular epoch (MJD 52 695.4). The flux is divided by the pseudo-continuum computed with ξ_c = ξ_l = 1. Two Gaussian profiles (red) represent the main component BAL₁ and BAL₂ of the BAL. The dotted line is the residual absorption corresponding to the blue NAL. The velocity scale is based on the systemic redshift z = 3.911.