Chris Curwen posted an article to TC-FORUM in 2001 (http://tc.eserver.org/23451.html) about the need for categories to clarify the field of technical communication. He goes to the lengths of arguing that we shouldn't use the term "technical communication" any more, until we define its constituent elements more clearly.
The TC Library website knows this need well. We currently use a four-level system of categories to index the thousands of works in our catalogue, a category scheme that we first developed in 2001. Our system is limited, and in 2003-04 we formed a special committee with Saul Carliner and JoAnna Springsteen to develop a definitive taxonomy for works indexed in our site. They were unable to accomplish this (the problem is that difficult), discouraging our site editors of the hope of ever developing useful categories.
I myself have given academic presentations about the difficulties of categorizing technical communication (http://tc.eserver.org/30577.html, http://lectures.eserver.org/1056). It's not an easy thing to do, if the journals and textbooks in our own field don't consistently agree as to the major and minor categories.
So. I'm hoping this discussion forum will encourage TC Library users to discuss the strengths and weaknesses of the system used on this website, and to suggest categories that might help us to improve our system (and the field's thinking as a whole about tech comm taxonomies).