A users unique ID, usually in the form of usr_c1644b5b-3ca4-45b4-97c6-a2a0de70d469. Legacy players can have old IDs in the form of 8JoV9XEdpo. The ID can never be changed.
The type of event that occurred. This is a string that is prefixed with the type of object that the event occurred on. For example, a group role update event would be prefixed with group.role.