Sources groups the citations and reference material this atlas draws its facts from, organized into Encyclopedia Article, the ordinary Wikipedia and reference-encyclopedia articles supporting the great majority of the atlas's sourced facts, and Internal System Reference, the small number of self-referential entries the atlas records rather than an external publication. A source entity carries the reliability tier, locator and quote evidence a fact cites back to. This category excludes the facts and properties a source supports, which live on the entities they describe, and the relation-type vocabulary used to connect an entity to its source, which belongs under Relation Types.