Sometimes, when you've made a custom submission template for a particular set of categories, you want to let people select a category 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 category for which you want itself and its descendants to be shown as options while excluding everything else. Place it in the template where the category selection takes place, replacing the previous category selection area.
This will work on the submit and edit listing pages and the submit and edit category pages.
Please note: This is not currently available for the level-based category selector, only the simpler one, so you'll have to switch off Admin Panel -> Settings -> Switches -> Level-based category selector. If you'd like to have it for the level-based selector please write to WSN support.