Left panel: cartoon of the distribution of the absorbing gas from the accretion disk plane. i is the angle between the line of site and the axial direction of the accretion disk, and . The gas column density is a constant within the angle width ∣β ∣ ≤σ. The BALs would be possibly observed along the line of site with small β, and the NALs would be possibly detected along the line of site with small i. Right panel: gas column density as a function of β, giving an angle width σ.