ACLs are a type of access control list. They are used to control the permissions of users and groups. An ACL is a set of instructions that define which users or groups have access to a certain resource. Those instructions can be stored in three different formats: as an ACL, as an Extended ACL, or as a named local group policy object (GPO). The first two formats can be applied within Windows NT file systems and NTFS file system volumes only. The last one is more flexible and can also be applied to Active Directory objects like user and group accounts, domains, organizational units (OUs), and sites.