Sketches of the emission of the targeted lines in the different phases of the ISM, for a PDR on the top and an XDR on the bottom. The incident photons, far-UV for PDRs and X-rays for XDRs, enter from the left. Thick dashed lines separate the different hydrogen phases: ionized (H⁺), neutral (H), and molecular (H₂). We do not show a scale regarding the depth of the cloud because it strongly depends on the ionization source and cloud properties. We note that in the XDR case, ionized transitions arise also where the gas is mostly neutral, and neutral carbon emission is distributed much further into the cloud since X-rays can penetrate much more deeply than far-UV photons.