Groups the citation sources the psychology atlas cites in support of its facts: encyclopedia entries, institutional pages, journal articles and similar. Sources are grouped as a type of their own, and a source's own SourceType classifies it further, Encyclopedia, Institutional Page, Journal Article and so on, without needing a subcategory tree of its own, since a source is bibliographic scaffolding rather than a browse subject in its own right. It deliberately excludes the psychologists, theories, studies and other subjects a source supports, each filed under its own type's category instead.