Classification of Broad Absorption Line (BAL) Quasars Based on Ionization State

Most BAL quasars predominantly show absorption from high-ionization transitions, such as N v, Si iv, and C iv (referred to as HiBALs, as mentioned by Weymann et al. in 1991). A smaller fraction, about 10% in optically selected samples, also exhibit absorption from low-ionization transitions, including Al ii, Al iii, and Mg ii (known as LoBALs, as reported by Voit et al. in 1993 and Gibson et al. in 2009).