When you make a setting to use in the ACP in the System Settings, for the Extra to show to the Admin, you put in #show_groups# or #show_forums#, etc, and then it gives you a list when you see the setting in use.
Like in Topis, Posts & Polls, the "Ignored Users: Protected Groups" setting, it shows a list of the different Member Groups. If you were to look at the data to make that setting possible, you would see #show_groups#
Make a new setting to play with: Setting Extra? #show_forums#
Enjoy! :D