Sorry for my (very) late answer, I do not think that you understood my mail. By the letters a, b, c I was refering to the content of my topics, not to the automatic lettering of the stylesheet.
My filter is supposed to hide the topic b, but it still appears when generating the PDF and I still get 3 topics instead of 2...
I think that the software (Arbortext Editor) is struggling to handle the filters at such sublevels (map>part>chapter>topicref>topicref).