  The Absorption Measure Distribution (AMD) in the X-ray outflows of Seyfert
active galactic nuclei (AGN) describes the distribution of absorbing column
density as a function of ionization parameter. Up to now, AMD has been measured
only for seven objects with high-resolution X-ray data that contain absorption
lines from ionized heavy elements. Even though the number of measured AMDs is
not large, they display a universal broad shape containing a prominent dip, for
which the absorbing column drops by around two orders of magnitude. In this
paper, we tested a range of photoionization models against the overall shape of
the AMD as observed in Seyferts. In particular, we demonstrate that the shape
of the AMD depends both on the spectral energy distribution (SED) of radiation
which enters the outflow, and the density of the warm absorber (WA). The model
that best reproduces the observed shape of the AMD is one wherein the gas
density of the WA is of the order of $10^{12}$ cm$^{-3}$, irradiated by an SED
whose optical/UV luminosity is 100 times higher than the X-ray luminosity. When
the cloud density is higher than $\sim 10^{11}$ cm$^{-3}$, free-free heating
dominates the entire absorber, and only one instability zone occurs, which is
in agreement with observations.
