Sketch illustrating how an ionizing spectrum hits the front of a cloud and gradually gets "eroded" as it moves into the cloud, with ionization edges appearing. Hence the ionizing field experienced by gas deeper in the cloud is weaker, and so it has lower ionization degree.
Changing fraction (y-axis) of carbon ions from the cloud front to back (x-axis, linear), for a black body ionizing radiation field (left) and a power law (AGN) ionizing radiation field (right). The AGN radiation field tends to generate a wider range of ionization degree than the BB radiation field. This also includes an extended partially ionized zone heated by X-rays (not shown in these figure).
Figures copied from Stasinska 1978 (left) and Davidson 1973 (right)