The category mechanism in ACS3x mostly just provides a framework for
customization. Some of the modules make use of the mechanism, i.e.
the mailing-list module, the todo module, the link manager module.
You can take a look at them for examples of how to use them.
Remember that the ACS is really a toolkit for website development, not
a website-in-a-box. As the toolkit matures, more and more
out-of-the-box capability is developed (wait 'til we get OpenACS 4 up
and running!). ACS 3.2 isn't very orthogonal, i.e. only part of the
modules are user group aware rather than all of them, etc. That's due
to the incremental nature of the development of the toolkit. ACS 4 is
among other things meant to be much more orthogonal, with the goal
being that all modules be subsite and group/party aware, that the
categorization mechanism be applicable to all content, etc...
