This read-create INTEGER, set per interface, holds the index of the ACL applied to that interface's egress traffic, with 0 meaning no ACL is applied. An admin uses this to filter traffic leaving a specific port, for example preventing certain traffic from exiting toward a particular downstream network segment. For example, an admin wanting to prevent internal management traffic from leaking out a customer-facing port would set this field to the index of an ACL that denies that management traffic on egress.