| Summary: | Supermassive black holes accreting matter at very high, perhaps even super-Eddington rates appear in the sky as a special class of luminous active galactic nuclei. The Eigenvector 1/quasar main sequence parameter space allows for the definition of easy-to-implement selection criteria in the rest-frame visual and UV spectral ranges. The systematic trends of the main sequence are believed to reflect a change in accretion modes: at high accretion rates, an optically thick, geometrically thick, advection-dominated accretion disk is expected to develop. Even if the physical processes occurring in advection-dominated accretion flows are still not fully understood, a robust inference from the models—supported by a wealth of observational data—is that these extreme quasars should radiate at maximum radiative efficiency for a given black hole mass. A key empirical result is that lines emitted by ionic species of low ionization are mainly broadened because of virial motions even in such extreme radiative conditions. “Virial luminosity” estimates from emission line widths then become possible, in analogy to the scaling laws defined for galaxies. In this contribution, we summarize aspects related to their structure and to the complex interplay between accretion flow and line emitting region, involving dynamics of the line emitting regions, metal content, and spectral energy distribution.
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