An entity affected by an alert. At least one of the fields other than activities will be non-null. The affected entity is the intersection of these fields, not the union: if stop and route both have values, the alert does not affect the entire route. See GTFS Realtime FeedMessage FeedEntity Alert EntitySelector. activities - The activities affected. direction_id - The direction of the affected trip. See GTFS trips.txt direction_id. facility - The facility affected. route - The route affected. See GTFS routes.txt route_id route_type - The type of route affected. If present alone, indicates the entire mode of transit is affected. See GTFS routes.txt route_type stop - The stop affected. See GTFS stops.txt stop_id * trip - The trip affected. See GTFS trips.txt trip_id