S3 Inventory and S3 Batch Operations add support to identify and copy objects to use S3 Bucket Keys, reducing the costs of Server-Side Encryption (SSE) with AWS Key Management Service (KMS). S3 Bucket Keys reduce the request costs of SSE-KMS by decreasing the request traffic from S3 to KMS. You can configure your bucket to use an S3 Bucket Key for AWS KMS-based encryption on new objects. With this update, you can use S3 Inventory and S3 Batch Operations to configure S3 Bucket Keys while creating encrypted copies of millions or billions of existing objects, reducing the cost of server-side encryption requests with AWS KMS.
Source:: Amazon AWS