With Amazon S3 Inventory, you can now easily review your access control lists (ACLs) on all of your objects to simplify review of access permissions. ACLs were the original way to manage object access when S3 launched in 2006. Now, when migrating to IAM-based bucket policies for access control, you can easily review all of the object ACLs in your buckets before enabling S3 Object Ownership.
Source:: Amazon AWS