I think I finally see what you are saying:

Even though a category in the database can only be associated with one category template, as long as that template exists within each template set, and has the same filename (eg. customcat.html), that category template can have completely different contents.

In other words, for those who are slow to get it like me :)

The script looks in the database at the given category, calls an assigned category template if there is one, (or defaults to categories.html), and depending on what template set has been called, loads the assigned category template, which, while it has the same filename, can be a completely different file in each template set...

Man, I'm sitting there looking at all these convoluted options when what I needed was right there! *smacks head*

Ah well, maybe it was the fumes from staining my deck this afternoon....

Thanks :^)

Roger