The meaning and contents of the data member depend on the event_id:
This is mainly used for events that are replies to (asynchronous) requests. It contains a status code, which is >= 0 on success, or < 0 on error (a mpv_error value). Usually, this will be set if an asynchronous request fails. Used for: MPV_EVENT_GET_PROPERTY_REPLY MPV_EVENT_SET_PROPERTY_REPLY MPV_EVENT_COMMAND_REPLY
One of mpv_event. Keep in mind that later ABI compatible releases might add new event types. These should be ignored by the API user.
If the event is in reply to a request (made with this API and this API handle), this is set to the reply_userdata parameter of the request call. Otherwise, this field is 0. Used for: MPV_EVENT_GET_PROPERTY_REPLY MPV_EVENT_SET_PROPERTY_REPLY MPV_EVENT_COMMAND_REPLY MPV_EVENT_PROPERTY_CHANGE MPV_EVENT_HOOK