rootid — Specify the root element to format
If rootid is specified, it must be the value of an ID that occurs in the document being formatted. The entire document will be loaded and parsed, but formatting will begin at the element identified, rather than at the root. For example, this allows you to process only chapter 4 of a book.
Because the entire document is available to the processor, automatic numbering, cross references, and other dependencies are correctly resolved.