Sometimes, when you've made a custom submission template for a particular set of forums, you want to let people select a forum but only within the applicable group. You can do that like this:
<select name="catid">{CATSELECTOR[26]}</select>
Change 26 to the id number of the forum for which you want itself and its descendants to be shown as options while excluding everything else. Place it in the template where the forum selection takes place, replacing the previous forum selection area.
This will work on the submit and edit topic pages and the submit and edit forum pages.
Please note: This is not currently available for the level-based forum selector, only the simpler one, so you'll have to switch off Admin Panel -> Settings -> Switches -> Level-based forum selector. If you'd like to have it for the level-based selector please write to WSN support.