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This is done in the document's Document Type Declaration, not in the DTD.
Bob DuCharme writes:
In a Document Type Declaration like this:<!DOCTYPE chapter SYSTEM "docbookx.dtd">
the whole point of the chapter part is to identify which of the element types declared in the specified DTD should be used as the root element. I believe the highest level element in DocBook is set, but I find it hard to imagine someone creating a document to represent a set of books. We are free to use set, book, chapter, article, or even para as the document element for a valid DocBook document.
[One job some parsers do is determine which element type[s] in a DTD are not contained in the content model of any other element type: these are by deduction the prime candidates for being default root elements. (PF)]
This is A Good Thing, because it adds flexibility to how the DTD is used. It's the reason that XML (and SGML) have lent themselves so well to electronic publishing systems in which different elements were mixed and matched to create different documents all conforming to the same DTD.
I've seen schema proposals that let you specify which of a schema's element types could be a document's root element, but after a quick look at section 3.3 of Part 1 of the W3C Schema Recommendation and the RELAX NG schema for RELAX, I don't believe that either of these let you do this. I could be wrong.