Another graphic question

Who created, or at least fully understands, this diagram?
http://docs.oasis-open.org/dita/dita/v1.3/csprd01/part3-all-inclusive/archSpec/base/document-type-shells.html I'm having trouble understanding two things, and I want to improve them in the revision.
Actually the two things are kinda both the same thing.
1. Is the constraint module deliberately blue (same as the DTD)?
2. Are the labels across the top meant to apply vertically to the rest of the components?
I.e. is the constraint module considered to be "part of" the document-type shell?
(It does seem correct to me that domain modules and structural modules are both considered "vocabulary modules".) mag