What is the difference between event_id and event_uid

The event_id is only used for events that your application creates, what we call managed events. This is an ID that your application specifies. We use it as a key for an application’s managed events.

The event_uid is a Cronofy wide unique key for the event. It is only used when interacting with external events.

For example when your app as obtained explicit permission from the end-user to edit external events you use the event_uid to edit external events