This parameter is only relevant when chunking is being performed, see Section 2.7, ““Chunked” output”. This parameter contains a list of XPath expressions. When chunking is being applied, for every element in the document, each expression is evaluated with that element as the context item. If the effective boolean value of any expression is true, the element is a candidate for chunking. It will become a chunk unless $chunk-exclude excludes it or, in the special case of recursive sections, if it is nested too deeply.

The default value for this parameter is:

('parent::db:set',
'parent::db:book',
'parent::db:part',
'parent::db:reference',
'self::db:refentry',
'self::db:section',
'self::db:sect1')

In other words all of the direct children of set, book, part, reference will become chunks (the special case of info is automatically excluded); refentry, section, and sect1 become chunks anywhere they appear. In the case of recursive sections, $chunk-section-depth also applies.

The namespaces in $chunk-filter-namespaces will be in-scope when this expression is evaluated.