This enables or disables Explicit Forward Congestion Indication (EFCI) marking of ATM cells on this port's Topaz I/O module. EFCI marking lets this specific port signal downstream congestion to receiving equipment so that ABR-style flow control can throttle sending sources before the port has to resort to outright cell discards, so enabling it can reduce loss during transient congestion at the cost of relying on far-end equipment actually honoring the marking. A port carrying ABR traffic to congestion-aware ATM equipment would typically have pportEFCIMarking enabled so that brief congestion on this port triggers rate reduction at the source rather than immediate cell loss.