This specifies whether a given profile attachment applies to traffic arriving on the interface (ingress) or leaving it (egress), a critical distinction since the same named ACL profile attached in different directions on the same or different interfaces enforces fundamentally different policy. An admin must be deliberate about this when designing filtering -- for instance, blocking a source subnet is typically an ingress concern, while restricting what a compromised internal host can talk to outbound is an egress concern. For example, an admin building defense-in-depth might attach one profile in the ingress direction to block known-bad external sources, and a separate profile in the egress direction on the same interface to limit what internal hosts behind it are allowed to reach outbound.