You can provide outlines inside the root object (but outside any page-sequences or
other formatting objects). Here's an example of an outline entry:
<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"
xmlns:fox="http://xml.apache.org/fop/extensions">
<fox:outline internal-destination="sec3">
<fox:label>Running FOP</fox:label>
<fox:outline internal-destination="sec3-1">
<fox:label>Prerequisites</fox:label>
</fox:outline>
<fox:outline>
</fo:root>
It works similarly to a basic-link. There is also an external-destination
property, but it isn't supported currently.