I think there is a typo in https://prgdoc.repetier-server.com/v1/docs/index.html#/en/web-api/websocket/index here:
All returned messages are in form of an object. First you see callback_id which is a reference to a command send. Since it is initiated by the server the reference is -1 meaning no reference. eventList means we got a list of events. When it is the servers time to send data it sends all events unsent so far. The events are contained in the data array. Each event is a single object having 3 fields:
- data - Payload of the event. Content depends on the event.
- event - Name of the event.
- data - Printer slug name the event belongs to.