The phrase "Comprehensive subject," is one that I've heard plenty of times, but a quick search on Google doesn't come up with many examples. My understanding is that the adjective comprehensive when related to the target noun subject is describing a:
- "Field" or "Area" of study
- Domain, Scene, etc
As containing/including an adequate amount of information to answer questions without having to extend outside that particular subject or field.
Of course you make the point of asking, "with respect to what?" which I think depends on the context in which it's used. Something that is considered comprehensive in one context, may not be comprehensive in another.
A Biology professor may choose a certain textbook over others because it is comprehensive with respect to the course materials, the goals of the course, and/or the topics within biology that are meant to be covered in that particular course.
Your quote from the text includes "Treatment of..." but doesn't say what that treatment is... which may be what the subject is referring to.