Category pages are virtual pages and are marked as invisible.
So the category are not displayed in the side menu.
Usually we have this sitemap :

• section 1
• section 2
  • subsection 2.1
  • subsection 2.2
  • classified ads (which is the root of classified ads
    • category 1 (virtual page)
      • ad 1
      • ad 2
      • ...
    • category 2 (virtual page)
      ...
      When I am displaying the page classified ads I'd like to have in the side menu all the child. But at this time it does not display the virtual page of classified ads because they are marked invisible.
      I'd like to have them visible.
      But let the ads invisible because we may have a lot of ads.