|$generate-nested-toc as xs:string := 'not(f:section(.)) 
  |                                     or (f:section(.) and f:section-depth(.) le $vp:section-toc-depth)'

Description

The $generate-nested-toc parameter is an XPath expression. If it evaluates to true when the context item is an element that appears in the table of contents (ToC), then the immediate children of that element will may also occur in the ToC.

For example, if this expression returns true for a chapter element that appears in the ToC, then the first level sections in that chapter will also appear in the ToC. If the expression returns true for one of those sections, then the second level sections of the chapter will appear in the ToC, etc.