Topic Maps assign no significance to the line between two nodes in most cases - not even "parent-child" or a "one to many" indicators (as in UML for example)
If the line was detailed by a relationship indicator that was itself defined by a SIG - then the nodes could have a much richer connection - even to the point of a mathematical one - or some logic code that would allow a much more informative view of the subject area than.
For the PM concept of a "work order" or the KM one of a "topic" - the trees could be expressed as:
<item 1><sig:key code>< item 2> (this would be a truple)
- which would not necessarily be reversible for the placement of topic1 and topic2.