Subroutes and itineraries are experimental. OSM route relations should ideally represent public-facing or service-level variants that passengers would recognize, but we do not yet have a stable rule for deriving those variants from OpenTransportData GTFS data. For now, the itinerary layer is only a heuristic reduction based mainly on GTFS headsign grouping, so these variant buckets are useful for inspection but should not yet be treated as a canonical OSM mapping target.