Specifies a jump to a labelled chapter, section, subsection subsubsection or figure.
The first argument is text to be highlighted (mouseable in help systems) and the second is the reference label. In linear documents, the section number is given following the text, unless the helprefn command is used instead, where the section number is suppressed.
An optional first argument in square brackets specifies a filename where the topic is found (not yet implemented).