The chapter on depth is really very long. The introduction to this section is a little to strong when read without that context, the division outlined here is one of many aspects of depth that gets discussed; you don't need to point out that there are others just as important:

For the purposes of designers, there are two major types of complexity that need to be considered. In abundant depth, a great deal of detail exists, but it often conforms to well-known patterns, creating a sort of “textural” paper which fills in another pattern. Baroque architecture, patterned wallpaper, lace, Corinthian capitals, tie-dye shirts, or a tile mosaic are all good examples of abundant depth. They are complex patterns, but can be understood fairly readily at a glance.

Abundant depth, while overwhelming, and preventing the mind from apprehending all of its details, can be understood readily because the mind can match the overall pattern. Our previous example of sitting in a cafe demonstrates this type of depth as well. The street-scape is complex, ever-changing, and overwhelming until one learns the overarching patterns. Once that occurs, the complexity of the street-scape reduces to a filler for the idea of “busy street”.

Refined depth, by contrast, tends not to be overwhelming in its operation, instead a phenomena largely conforms to a pattern, with only a slight tweaking suggesting something unknown exists. The plaza of the Salk Institute, a Samurai sword, the Parthenon of Athens, Venturi's '50s diner, or the smile of the Mona Lisa are good examples of refined depth. In each case, the phenomena is not particularly complex seeming, they are an empty courtyard, a big knife, a columned temple, a '50s diner, and a portrait of a lady. But in each case there is something not-quite normal about the phenomena, something which causes the mind to reject it as a simple match for the pattern and search for some other explanation (or reject the pattern as uninteresting/foreign/unimportant).

In refined depth the mind can apprehend the pattern easily, but is left feeling that something is missing, hidden, unclear or unseen, something which is controlling the environment. This feeling that something is almost-understood is a commonly used tool for designers. In fact the use of the “hidden” or “unseen” mover in art has been so commonly used that many users have become jaded regarding the largely arbitrary actions by artists and will tend to discount art which uses simplistic methods of constructing refined-seeming depth.

One interesting example of the effect of the hidden or unseen mover in design is ritualistic behaviour. Here a largely arbitrary-seeming system of actions are performed which are based on a liturgical framework. The liturgy becomes the hidden actor which controls the actions of the participants. For outsiders, the result seems largely arbitrary, but they can see large numbers of people making similar or interlocking actions, the ritual will seem foreign, often scary and fascinating. For insiders, the ritual is comforting and familiar, based as it is on their shared liturgy. They can “see” the hidden actor which is made manifest through their actions.

Being someone who “knows the secret” is attractive to the individual, for the same reason that we investigate the unknown patterns in our environment in the first place. When something is not merely unknown, but actively hidden, the desire to understand by outsiders is normally greater. Secret societies, handshakes and decoder rings all hold a fascination for most users. Of course, as with any pattern, repeated disappointment will tend to dull interest. Discovering that most secret societies are just social clubs, and that decoder rings are mostly rotor ciphers will tend to make individuals discount the potential of any value arising from belonging to other groups.