Kenneth, people who want to use at least some of OpenACS just about
always install all of the OpenACS core, plus whatever additional
packages they're interested in.
There is some interest in refactoring a bunch of the OpenACS Tcl code
(e.g., the database API, templating system, and user authentication)
to make it easier to use in non-OpenACS AOLserver environments, but
currently it's not especially easy to do, and it definitely won't work
that way out of the box.
So unless you have unusual or extreme requirements (e.g., an
application server with no human users and which must not use a
database at all), I recomend just installing the OpenACS core and
using it, rather than trying to extract medium-sized chunks of
functionality from the toolkit in order to use them stand-alone in
AOLserver.
Once you are more familiar and proficient with OpenACS, you will
probably see for yourself those few applications where you really
might be better of just using some code taken from OpenACS, rather
than installing OpenACS. But those areas are probably much fewer than
you'd at first think, and hardly anyone does it, so you should
definitely start out with the "mainstream" path of installing the
OpenACS core and using it.