SubscriptionName: [required] The name of the Amazon DocumentDB event notification subscription.
SnsTopicArn: The Amazon Resource Name (ARN) of the SNS topic created for event notification. The ARN is created by Amazon SNS when you create a topic and subscribe to it.
SourceType: The type of source that is generating the events. For example, if you want to be notified of events generated by an instance, set this parameter to db-instance. If this value is not specified, all events are returned.