From Online Manual
Create New Category
On this page you can create a new category on your forum. Just like when you modify an existing Category you need to supply a name for the category, choose a placement for the whole Category using the order listbox and choose if it can 'collapse". "Collapsing" means that a Category can be suppressed to only show the title of itself, hiding any of its boards underneath. Clicking on the Category title of a currently collapsed Category usually opens the display of boards again underneath that category.