_        _______           _______  _          _______ 
( \      (  ____ \|\     /|(  ____ \( \        / ___   )
| (      | (    \/| )   ( || (    \/| (        \/   )  |
| |      | (__    | |   | || (__    | |            /   )
| |      |  __)   ( (   ) )|  __)   | |          _/   / 
| |      | (       \ \_/ / | (      | |         /   _/  
| (____/\| (____/\  \   /  | (____/\| (____/\  (   (__/\
(_______/(_______/   \_/   (_______/(_______/  \_______/

S3 ACL is a sub-resource attached to every S3 bucket and object. It defines which AWS accounts or groups are granted access and the type of access.

Important: AWS recommends using S3 bucket policies or IAM policies for access control. S3 ACL is a legacy access control mechanism that predates IAM. However, if you already use S3 ACLs and you find them sufficient, there is no need to change.

Hint 3
ACL is attached to individual objects within the bucket. 

"Grants": [
    {
        "Grantee": {
            "Type": "Group",
            "URI": "http://acs.amazonaws.com/groups/global/AllUsers"
        },
        "Permission": "READ"
    }
]