Add references to GTFS-Static

gtfs-realtime/spec/en/reference.md

@@ -605,7 +605,7 @@ Represent a new Stop added to the feed dynamically. All fields are as described
 
 ## _message_ TripModifications
 
-A `TripModifications` message identifies a list of similar `trip_ids` sharing the same stop pattern, which are all affected by a particular modification, like a detour. The modification is in effect on all of the listed service_dates, until it is removed from the feed. On any given service date, a trip MUST NOT be assigned to more than one `TripModifications` object. 
+A `TripModifications` message identifies a list of similar `trip_ids` from the GTFS-Static sharing the same stop pattern, which are all affected by a particular modification, like a detour. The modification is in effect on all of the listed service_dates, until it is removed from the feed. On any given service date, a trip MUST NOT be assigned to more than one `TripModifications` object. 
 
 There MAY be multiple `TripModifications` for a given stop pattern. It may be desirable to split the trips into multiple modifications e.g. if the `propagated_modification_delay` changes significantly, over the course of the detour.
 
@@ -621,7 +621,7 @@ The frequency of data updates is expected to be approximately hourly (~24 times/
 | _**Field Name**_ | _**Type**_ | _**Required**_ | _**Cardinality**_ | _**Description**_ |
 |------------------|------------|----------------|-------------------|-------------------|
 | **modifications_id** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Required | One | An arbitrary string to identify this detour. The `modifications_id` MUST be unique for all currently active detours. The ID of each replacement trip is generated by concatenating `modifications_id` + ‘_’ + `trip_id`. |
-| **trip_ids** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Required | Many | A list of trips affected by this detour. All trips linked by those trip_ids MUST have the same stop pattern. Two trips have the same stop pattern, if they visit the same stops in the same order, and have identical stop sequences. If the value `start_times` is set, a maximum of one trip_id can be listed.  |
+| **trip_ids** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Required | Many | A list of trips_ids from the GTFS-Static affected by this detour. All trips linked by those trip_ids MUST have the same stop pattern. Two trips have the same stop pattern, if they visit the same stops in the same order, and have identical stop sequences. If the value `start_times` is set, a maximum of one trip_id can be listed.  |
 | **start_times** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Optional | Many | A list of start times in the real-time trip descriptor for the trip_id defined in trip_ids. Useful to target multiple departures of a trip_id in a frequency-based trip.  |
 | **service_dates** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Required | Many | Dates on which the detour occurs, in the YYYYMMDD format. A trip_id will only be modified if it runs on a given service date; the trip IS NOT required to run on all of the service dates. Producers SHOULD only transmit detours occurring within the next week. |
 | **shape_id** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Optional | One | The ID of the new shape for the modified trips. May refer to a new shape added using a GTFS-RT Shape message, or to an existing shape defined in the GTFS-Static feed’s shapes.txt. |
@@ -644,7 +644,7 @@ _Propagated detour delays affect all stops following the end of a modification.
 
 | _**Field Name**_ | _**Type**_ | _**Required**_ | _**Cardinality**_ | _**Description**_ |
 |------------------|------------|----------------|-------------------|-------------------|
-| **start_stop_sequence** | [uint32](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Required | One | The first stop sequence of the original trip that is to be affected by this modification. |
+| **start_stop_sequence** | [uint32](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Required | One | The stop sequence value from the GTFS-Static of the first stop of the original trip that is to be affected by this modification. |
 | **num_stops_replaced** | [uint32](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Required | One | The number of stops which are canceled and replaced by the `new_stops`. May be zero to indicate no skipped stops. |
 | **propagated_modification_delay** | [int32](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Optional | One | The number of seconds of delay to add to all departure and arrival times following the end of this modification. Can be a positive or negative number. If multiple modifications apply to the same trip, the delays accumulate as the trip advances. <br/><br/>If the value is not supplied, consumers MAY interpolate or infer the `propagated_modification_delay` based on other data.  |
 | **new_stops** | [ReplacementStop](#message-replacementstop) | Optional | Many | A list of replacement stops, replacing those of the original trip. The length of the new stop times may be less, the same, or greater than the number of replaced stop times. |