Presence conditions applicable to fields and files:

* **Required** - The field or file must be included in the dataset and contain a valid value for each record.

* **Optional** - The field or file may be omitted from the dataset.

* **Conditionally Required** - The field or file must be included under conditions outlined in the field or file description.

Signs applicable to Float or Integer field types:

* **Non-negative** - Greater than or equal to 0.

* **Non-zero** - Not equal to 0.

* **Positive** - Greater than 0.

When calculating an itinerary, GTFS-consuming applications interpolate transfers based on allowable time and stop proximity. [Transfers.txt](#transferstxt) specifies additional rules and overrides for selected transfers.

Fields `from_trip_id`, `to_trip_id`, `from_route_id` and `to_route_id` allow higher orders of specificity for transfer rules. Along with `from_stop_id` and `to_stop_id`, the ranking of specificity is as follows:

1. Both `trip_id`s defined: `from_trip_id` and `to_trip_id`.

2. One `trip_id` and `route_id` set defined: (`from_trip_id` and `to_route_id`) or (`from_route_id` and `to_trip_id`).

3. One `trip_id` defined: `from_trip_id` or `to_trip_id`.

To navigate from the station entrance/exit (a node represented as a location with `location_type=2`) to a platform (a node represented as a location with `location_type=0` or empty), the rider will move through walkways, fare gates, stairs, and other edges represented as pathways. Generic nodes (nodes represented with `location_type=3`) can be used to connect pathways throughout a station.

Pathways must be defined exhaustively in a station. If any pathways are defined, it is assumed that all pathways throughout the station are represented. Therefore, the following guidelines apply:

- No dangling locations: If any location within a station has a pathway, then all locations within that station should have pathways, except for platforms that have boarding areas (`location_type=4`, see guideline below).

- No pathways for a platform with boarding areas: A platform (`location_type=0` or empty) that has boarding areas (`location_type=4`) is treated as a parent object, not a point. In such cases, the platform must not have pathways assigned. All pathways should be assigned for each of the platform's boarding areas.

- No locked platforms: Each platform (`location_type=0` or empty) or boarding area (`location_type=4`) must be connected to at least one entrance/exit (`location_type=2`) via some chain of pathways. Stations not allowing a pathway to the outside of the station from a given platform are rare.