The olink element allows to create links between different documents. Once the olink element has been inserted in a document, you have to specify a value for its targetdoc attribute and optionally, a value for its targetptr attribute. The targetdoc attribute contains the symbolic name of the document which is the target of the olink. The targetptr attribute is the ID of an element found in the target document. More information about the olink element and how this element is processed by the DocBook XSL stylesheets in DocBook XSL: The Complete Guide, by Bob Stayton.