From the Introduction:
This Gazetteer is a game aid intended to assist a Games Master in using Hell as an adventuring location in their campaign. It briefly describes a large number of locations that may be of interest to Adventurers, or that may be referred to by the Games Master as background, historical, or reference material. The Gazetteer includes maps showing the locations of each described location.
This product is a Games Master’s reference that describes in detail a portion of the geography of Malebolge (Inferno), in particular the Third and Fourth Circles, located in central Hell. It is an overview of a part of the middle section of the Pit. A 'Circle' is a toroid-shaped (doughnut-shaped) region of Hell, with a fixed inner and outer radius relative to Hell's fixed geographical center. Each Circle is ruled by a Diabolic Prince with his unique Dukes or Earls, guarded by Infernal Legions commanded by Diabolic Generals, is home to a specific group of Devils, populated by a particular class of sinners. Many Circles are separated from their inner and outer neighbors by cliffs, swamps, and rough terrain.