How to properly categorise an article in OSHWiki
See Categories for detailed information about MediaWiki categories.
When editing the article you write the category between square brackets e.g. http://oshwiki.eu/en/index.php/Working_time
You can add a new category by searching for it e.g. Category:MyNewCategory and then clicking on the red link to create it.
To create a subcategory, you add the category as normal (e.g. search for Category:Subcategory) then edit the subcategory and add the parent category [[Category:Parent_Category]. For example, the Legislation category is a sub-category of "OSH in general":
While it is possible to browse through the tree of categories to find each article, it is best to explicitly add all parent categories to the article so that they appear at each level. For example, if you would like an article about general OSH legislation concerns to appear both under Legislation and "OSH in general" you should add the following to the content of the article: