Names for methods of categorizing?

[Nick Arnett]

I believe there was a brief discussion here regarding terminology for how
information resources become categorized.  Logicians use the terms
"intension" to describe rules for membership in a set and "extension" to
describe the fact of membership.  (Please correct me if I'm wrong here,
these words are confusing, even though -- or perhaps because -- my father
taught logic.)

Someone, I believe on this list, mentioned a similar pair of terms that
librarians commonly use to described cataloging by rule or by...?  I'm
seeking those terms, or any other clear ways to describe these two aspects
of categorization.

In case you're curious, I'm working on a white paper describing new aspects
of our software, which presently can organize information by rules, which we
call Topics or, as the world tends to call them, queries.  We're adding the
ability to organize information manually, to correct errors that the rules
generate or to categorize in ways that are not amenable to computers:
"things to do tomorrow," "well-written articles," "humorous articles," etc.
These are categories for which we often can't come up with an explanation,
much less write rules so that software could assist us by automating the
process.

Your help will be appreciated!

Nick Arnett
--
    Senior Product Manager, Knowledge Applications
        Verity Inc.  (http://www.verity.com/)
        "Connecting People with Information"
  Phone: (408) 542-2164  E-mail: narnett at verity.com