Is there a description of this functionality anywhere, please?
Well, I guess the lack of answers is an answer in itself.
You guessed correctly - not that I would know about it. Something (http://simutrans-germany.com/wiki/wiki/tiki-index.php?page_ref_id=1014) is on wiki but that's nearly nothing. If you're asking about the colourful buttons, your guess will be as good as mine: try and see for yourself :) Perhaps I'll add the little bits that I know to the page...
There is a working help button for this dialogue too, in the upper left corner ...