The conventional POSIX permission concept uses three classes of users for assigning permissions in the file system: the owner, the owning group, and other users. Three permission bits can be set for each user class, giving permission to read (r), write (w), and execute (x). An introduction to the user concept in Linux is provided in the User Guide.
Default ACLs can only be applied to directories. They determine the permissions a file system object inherits from its parent directory when it is created.
Each ACL consists of a set of ACL entries. An ACL entry contains a type (see Table B.1. “ACL Entry Types”), a qualifier for the user or group to which the entry refers, and a set of permissions. For some entry types, the qualifier for the group or users is undefined.