The JSON Training/Track Format (JTT) is intended to be a JSON-based alternative to the existing XML-based formats for training and GPS data. The two most relevant existing formats in that space are the GPS Exchange Format (GPX) and Training Center Database XML (TCX).
Some parts of the format may require extensible vocabularies (such as activity types and data types), which may end up being managed via a registry and extensibility rules (such as the ones defined in RFC 5988). Activity types and data types are currently being considered for being managed via a registry.