Chapters 3 through 8 go into XML and some standards built on basic XML in considerable depth. Depending on how familiar you are with XML and how deeply you want to get into the subject, you may be able to skip or skim much of Part II. You can always refer back to it if you run into a question later in this book.
Chapter 3 offers some general background on XML and XML Namespaces, except for DTDs (which are covered in Chapter 4). Namespaces permit mixing XML for different sources by qualifying the names that appear.
Chapter 5 focuses on XML schemas. Both DTDs and schemas are means of specifying the allowed syntax of an application. DTDs, which were part of the original definition of XML, are covered in great depth. Schemas, a more recent addition to XML, are not covered as thoroughly but enough information is given that you will be able to understand the uses of schemas in the XML Security standards specifications. Although schemas do not completely replace DTDs, they are better adapted to use with namespaces and are used for the authoritative syntax specification in the XML Security standards.
Chapter 6 covers XPath in great depth. The XPath data model is used through out XML Security including canonicalization, signature, and encryption (Parts III and V). Nevertheless, a full implementation of XPath is not required as part of XML Security. Unless you are very familiar with XPath, you should read most of Chapter 6. You probably don't need to study the entire function library described in Section 6.5, however.
Chapter 7 examines Uniform Resource Identifiers (URIs), the standard way to identify information and resources on the Internet, xml:base, which provides a means to the base from which relative URIs are interpreted, and XPointer, a powerful syntax for specifying pieces of information within larger XML aggregates.
Finally, Chapter 8 gives a snapshot of current state of the work involved in developing an XML Protocol framework.