This is a "tree of all knowledge" category, a top-level place to start when browsing Wikipedia categories for articles. This is the top level in terms of encyclopedia article function and content. It is intended to contain all and only the few most fundamental ontological categories which can reasonably be expected to contain every possible Wikipedia article under their category trees. These categories are: physical entities; biological entities; social entities; and intellectual entities.
An alternative root category, based on a somewhat more detailed initial classification, is Category:Main topic classifications.