Thanks for the reply. The explanation of 0 being essentially/pragmatically the top category makes perfect sense.

I have been trying to understand the workings of the centerbox for a while, both in the default Zen Cart and with Numinix's module, and although I have posted a much reduced form of my specific problem to the Numinix forum, your simple comment has helped me a lot with understanding the workings of Zen Cart, and is much appreciated.