An access-control list (ACL) is a list of permissions
associated with a system resource (object). An ACL specifies which
users or system processes are granted access to objects, as well
as what operations are allowed on given objects. Each entry in a
typical ACL specifies a subject and an operation. For instance,
if a file object has an ACL that contains (Admin: read, write; guest
1: read), this would give Admin permission to read and write the
file, and only give guest 1 permission to read it.