Defines how documents should be delivered to Amazon S3. When set to FailedDocumentsOnly, Firehose writes any documents that could not be indexed to the configured Amazon S3 destination, with elasticsearch-failed/ appended to the key prefix. When set to AllDocuments, Firehose delivers all incoming records to Amazon S3, and also writes failed documents with elasticsearch-failed/ appended to the prefix. For more information, see Amazon S3 Backup for Amazon Elasticsearch Service Destination (http://docs.aws.amazon.com/firehose/latest/dev/basic-deliver.html#es-s3-backup). Default value is FailedDocumentsOnly.