  When an AGN is obscured, the warm reflecting gas nearby can be seen by a
combination of bremsstrahlung, intrinsic line emission, and reflection of the
nuclear continuum, both by electron scattering and by resonance line
scattering. Resonance lines, due both to intrinsic emission and scattering, are
particularly prominent in the soft X-ray band.
  When our line of sight to the nucleus is not obscured, the dominant effect is
absorption. In the soft X-ray band, ionization edges of highly ionized species
and resonance lines contribute comparably to the opacity; in the ultraviolet,
the gas is almost transparent except for a small number of resonance lines. We
identify the "warm absorbers" seen in many AGN X-ray spectra with this gas, but
argue that most of the UV absorption lines seen must be due to a small amount
of more weakly ionized gas which is embedded in the main body of the warm,
reflecting gas. Because the ionization equilibration timescales of some ions
may be as long as the variability timescales in AGN, the ionic abundances
indicated by the transmission spectra may not be well-described by ionization
equilibrium.
