PDF files can optionally include an active table of contents that is displayed alongside the pages in the PDF viewer. These references are called bookmarks.

PDF bookmarks are generated using special functions in each XSL-FO processor because XSL cannot produce them directly. So if you are using Apache FOP, for example, you need to turn on the FOP extension functions by setting the fop.extensions parameter to 1. For example:

xsltproc  --output myfile.fo  \
  --stringparam  fop.extensions  1 \
  fo/docbook.xsl  myfile.xml

If you are instead using Antenna House's XSL Formatter, you should set the axf.extensions parameter to 1, or if you are using RenderX's XEP you should set the xep.extensions parameter to 1. When you turn on one of these extensions, special FO elements are written by the DocBook XSL stylesheets to the FO file. These special elements use the fox: namespace for FOP, the axf: namespace for Antenna House, and the rx: namespace for XEP. Those special FO elements are only recognized by the specific processor being used. If you turn on the wrong extensions, you probably won't be successful.