In Docbook 4, the toc and other nav elements (glossary, bibliography, index) were allowed at both the beginning of sections and the end. In DocBook 5, they are allowed only at the end of section, sect1, etc. They are allowed in both locations in chapter, appendix, and preface, as well as topic. Certainly the toc element at least should be allowed at the beginning of sections. This change would be backwards compatible since it loosens the content model.