Categories can have their display ability turned on or off.
When a Categories is created via the article editor its display property is set to be visible by default.
Categories can now be set to visible or invisible with a show property. When a Category is set to be visible it will be listed in the Site map and Category maps, as well articles in this Category are able to be searched by visitors and the results displayed.
When this property is set to invisible the Category is no longer able to be searched or displayed in the site map, category map or search results.