A frontmatter (note: Frontmatter) domain names no domain (note: Domain) declared under domains: of the configuration (note: Configuration), by its identifier (note: Identifier) or by its identifier path. The frontmatter wins over the folders and the globs (21 passages, no note), so the value (35 passages, no note) is kept as written on the entity (note: Entity) and the site (note: Site) shows what the author (note: Author) meant, but nothing files the note (note: Note) under a declared domain. Distinct from the unclassified domain, where the note names nothing, sits under no claimed folder and matches (note: Occurrence) no glob.
Check (note: Check) W-DOMAIN-UNKNOWN, severity (note: Severity) warning by default, documented with a before and after example in the check pages. The severity can be overridden per project (note: Project) or per repository (note: Knowledge repository).