Opaque DXN custom tag (@tag value), produced when no decoder is registered for its name (Dextrin.Registry.put_tag/3) — the open extension point DXN.md §1.2 reserves for any tag name that isn't one of the built-ins (@uuid, @duration, ...). Prefer a schema-backed struct over a custom tag for anything with real field structure; reserve custom tags for simple scalar-wrapping.