Broad absorption line quasars are quasars which show broad, blueshifted aborption troughs in their rest frame optical/UV spectra, which indicates outflows along the line of sight. These outflows are AGN-driven, but it is still an open question whether they exist in every quasar, and we only see them in a small fraction because they have a limited opening angle; or if they represent a specific, short-lived phase in a quasar's life.  

Broad absorption line quasars are remarkably similar to quasars without these absorption lines in almost every waveband except radio.

Morabito et al. 2019, A&A, 622, 15 showed that the radio detection fraction of brad absorption line quasars is correlated with the balnicity index (a proxy for "strength") of the absorption trough; see the figure on the left.