Property:Has page type
The schema of a page, declared by its infobox template (the infobox IS the schema declaration): doc_archive for archived documents (Template:Infobox doc_archive); future infobox types (person, organization, …) declare their own value.
One fact per page. Consumers dispatch on it (which property list / renderer applies), queries and batch jobs scope with it ([[Has page type::doc_archive]]), and it disambiguates property names shared across schemas (e.g. Has organization).