In SMF a category is just a container for boards, i.e. you can not post anything in a category, you can only post in a Board or a sub-board. SMF allows for unlimited sub-boards.
In Discourse you can create topics in categories and sub-categories (two levels). To dig more, you use tags.
Hi Rafael, maybe the “Archives” name is not a good example. I just wanted to know what exactly happens with sub-sub-sub-category from SMF or any other traditional bulletin board when migrated to Discourse. Thanks.