Explanation:
The JSON object for an AWS Config event contains an invoking Event attribute, which describes the event that triggers the evaluation for a rule. If the event is published in response to a resource configuration change, the value for this attribute is a string that contains a JSON configuration Item or a configuration Item Summary (for oversized configuration items). The configuration item represents the state of the resource at the moment that AWS Config detected the change. If the event is published for a periodic evaluation, the value is a string that contains a JSON object. The object includes information about the evaluation that was triggered. For each type of event, a function must parse the string with a JSON parser to be able to evaluate its contents.
Reference: http://docs.aws.amazon.com/config/latest/developerguide/evaluate-config_develop-rules_example-events.html