process.empty.source.toc — Generate automated TOC if toc element occurs in a source document?
Specifies that if an empty toc element is
found in a source document, an automated TOC is generated.
Depending on what the value of the
  generate.toc parameter is, setting this
  parameter to 1 could result in generation of
  duplicate automated TOCs. So the
  process.empty.source.toc is primarily useful
  as an "override": by placing an empty toc in your
  document and setting this parameter to 1, you can
  force a TOC to be generated even if generate.toc
  says not to.