In Wixi, every ''folder'' in the tree is a category. Adding pages to a category can be done by right-clicking on a category. Select the "append to page" option. This will append a rule to the page with a category mark. You may add more then one category.
Note that if you are adding more than one category you may do this in one line by using a comma after the first category.
For example:
%!wixi-category: About, Info, Etc
Or you can use this methode:
%!wixi-category: About %!wixi-category: Info