ACL

ACL stands for access control list and is used to control who can access S3 buckets and the objects inside those buckets. While bucket policies are usually reserved for controlling access for an entire bucket, ACLs (when used in the context of S3) can be used to control access to individual S3 objects, they give you “fine grained” control over resources in the bucket. You can find an explanation on the differences between bucket policy, IAM and ACLs here.

They are also considered a legacy service.