The GTFS reference defines all files, fields, and relationships of GTFS datasets. There are currently two official reference documents:
The GTFS reference documentation is not set in stone. It is an open specification developed and maintained by the community of transit agencies, developers, and other stakeholders who use GTFS. This community of producers and consumers of GTFS data distributes and discusses proposals to extending the spec to enable new capabilities, some of which are implemented in the GTFS reference. To help manage that process, procedures and guidelines have been established. See GTFS Amendment Process.