I think this is best answered in Section 2.1 (Laying out the page / Introduction). I'll quote the paragraph here:
There are three main parts to a page: the page itself, the typeblock,
and the margins separating the typeblock from the edges of the page.
Of slightly lesser importance are the running headers and footers, and
possibly marginal notes. The art of page design is obtaining a
harmonious balance or rhythm between all these.
In other words, the typeblock is the rectangle inside which all the body of the document is contained. It does not include running headers and footers, nor margin-, side-, and footnotes.
It may also help you to read Peter Wilson's excellent companion book, A Few Notes on Book Design, especially section 3.2. Again, I quote:
A page in a book will typically contain several elements. Principal
among these is the typeblock, but there are also items like the folio
(that is, the page number), a running header and/or footer which
carries the chapter and/or book title, and possibly marginalia and
footnotes. These latter elements, although essential to the content of
the book, are minor visual elements compared to the typeblock.