|The map with 40 notable locations|
Aside from being a tool to create maps, the Chthonotron also is the most concise and direct setting description in the three volumes comprising Chthonic Codex. Twelve terse pages (ten for for the Chthonotron proper, two additional ones for the random encounter tables that are used in some cases by the former) cover the hypogea's make-up in a handful of short tables and the description of the entries.
The tables themselves are a compressed shorthand about the mundane (ruins, fortifications, caves, lakes, springs, etc), chthonic (chasms, monster dens) and mystic/magic (altars, secret places, mana-tar seams) features of the Hypogea, and their descriptions grant us an insight into the inner workings of those features: We learn that there is a duality of gods and offspring of Erebus (also called chthonic deities), that Bocklin Polycerates know the secret locations of mana-tar seams which may or may not be on the map, that the Stargazers build observatories to look at the stars, that beyond the schools there are monasteries, hermitages, hidden treasure, lakes, mires, and many more things one wouldn't neccessarily expect in the cavernous karst that you are creating.
The tables do refer back to other tables in the Chthonic Codex (i.e., the laws of reality are referenced in some place, others tell you to find out what hidden other map feature can be found), and others have procedures to detail them; the simple result of Catacombs for example provides a page worth of more rules to build entire Necropolises with dozens of rooms.
Some of these features are hidden, or hard to find, from simply well-out-of-the-way hermitages to places important to learn secret mysteries which are well guarded in addition to their otherwise unusual ways to access; and of course some of the villages of the humanoid and cephalopod inhabitants of the Hypogean that aren't affiliated with, or in some cases outright enemies of the Great Schools. For this reason I would suggest referees make a copy of the map before adding possibly-secret entries to easily provide a map with known-only features. After all, the player characters are supposed to have grown up or living in the Hypogea for a while, so they would know a couple details about their environment; with others lying in wait to be discovered, or to spring into action at a referee's whim.
As I mentioned in the first post about the Chthonotron: What it doesn't tell you is why these things fit together. In fact, the author urges the referee to make sense of seemingly-unusual or badly-fitting results, in three ways: Explain them as fixed features of the map, explain them with current events, or just let it be for the players to figure out when they get to it. So the Chthonotron doesn't do all the work for you, but from my point of view it gives a referee space to breathe exactly where needed to make the campaign environment your own.
If you are spoiler resistant, check out the complete map key which includes all features of the map (but not all built out to their full detail). For me it paints a clear picture of what's going on in the Hypogea, and which groups are in power, and which ones might struggle for recognition or survival ...