  Collecting and analysing X-ray photons over either spatial or temporal scales
encompassing varying optical depth values requires knowledge about the optical
depth distribution. In the case of sufficiently broad optical depth
distribution, assuming a single column density value leads to a misleading
interpretation of the source emission properties, nominally its spectral model.
We present a model description for the interstellar medium absorption in X-ray
spectra at moderate energy resolution, extracted over spatial or temporal
regions encompassing a set of independent column densities. The absorption
model (named disnht) approximates the distribution with a lognormal one and is
presented in table format. The solution table and source code are made
available and can be further generalized or tailored for arbitrary optical
depth distributions encompassed by the extraction region. The disnht absorption
model presented and its generalized solution are expected to be relevant for
present and upcoming large angular scale analyses of diffuse X-ray emission,
such as the ones from the extended ROentgen Survey with an Imaging Telescope
Array (eROSITA) and the future Athena missions.
