When creating an article you can categorise them by selecting an article type. The list of article types are not stored in the category tree. If you have the correct privileges, you are able to create these on the fly by clicking on the article link hyperlink on the article window when the article type field is blank.
NOTE: From version 7.2 onwards you are able to remove the hyperlink on the window. So if you find that you do not have a hyperlink on the Article Type label then you need to edit the properties of this label in Window Manager and set the Hyperlink = True.
NOTE: It is possible to create article types using a script but this must be fully tested on your test system and a backup taken before applying to live:
insert into km_article_type
values (newid(), ' Article name ')