  A landmark of accretion processes in active galactic nuclei (AGN) is the
continuum originating from a complex structure, i.e., an accretion disk and a
corona around a supermassive black hole. Modelling the broad-band spectral
energy distribution (SED) effectively ionizing the gas-rich broad emission line
region (BLR) is key to understanding the various radiative processes at play
and their importance that eventually leads to the emission from diverse
physical conditions. Photoionization codes are a useful tool to investigate two
aspects, the importance of the shape of the spectral energy distribution, and
the physical conditions in the broad emission line region. In this work, we
critically review long-standing issues pertaining to the spectral energy
distribution shape and the anisotropic continuum radiation from the central
regions around the accreting supermassive black holes (a few 10-100
gravitational radii), with a focus on black holes accreting at high rates,
possibly much above the Eddington limit. The anisotropic emission is a direct
consequence of the development of a geometrically and optically thick structure
at regions very close to the black hole due to a marked increase in the
accretion rates. The analysis presented in this paper took advantage of the
look at the diversity of the type-1 active galactic nuclei provided by the main
sequence of quasars. The main sequence permitted us to assess the importance of
the Eddington ratio and hence to locate the super Eddington sources in
observational parameter space, as well as to constrain the distinctive physical
conditions of their line-emitting BLR. This feat is posing the basis for the
exploitation of quasars as cosmological distance indicators, hopefully allowing
us to use the fascinating super Eddington quasars up to unprecedented
distances.
