Hidden categories are those that have been tagged with the magic word __HIDDENCAT__.
The categories which should appear here are the maintenance categories, that is, categories reflecting the present status of the article, rather than classifying the article subject.
Hidden categories do not appear listed on articles. If you are wondering why the category you have added to an article has not appeared, check to see if the category is hidden. Previewing a page will list hidden categories at the bottom of the page, below the list of templates used.
Hidden categories are not hidden on category pages.
You can view hidden categories by clicking the box to "Show hidden categories", under the "Misc" tab at "your preferences". This will show them on a separate line, marked as "Hidden categories".
This category has the following 20 subcategories, out of 20 total.