Very often you need to highlight menu items or other markers depending on your location. With the index page, forums etc it is pretty much simple:
<!-- IF {PHP.z} == index --> class="selected"<!-- ENDIF -->
Pages and lists, however are more complex. You might have something like this:
... [ Articles ] [ Blogs ] ...
And each cat would have subcats, subsubcats and, of course, pages.
<!-- IF {PHP.z} == pages AND {PHP.c} == articles --><!-- ENDIF -->
and adding...