If I pick a specific subject in a scene that I want to print as zone III on paper it is very possible because an underexposure to get a negative with no usable info for that subject, no detail just black. Its a demonstration of the classic advice we all get that with an underexposure detail is truly lost.
With that same subject defining zone III the other three curves have zone III but at different densities.
Defining or pegging print zones in my example is very much done as Adams might have, looking at a scene, deciding what range to pick and picking certain subject matter and saying I want that to fall in such and such zone.
Printing the curves I labled as box and extra would require different enlarger exposures.
The value I see in consistent negative density for a given zone is not addressed by my illustration. That is more a matter of working efficiently.