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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.