This octet-string bitmask is the extended-width equivalent of rcEgressMirrorPorts, selecting which ports have their egress traffic mirrored, used on switches with more ports than the standard PortList type can address. Admins use it instead of rcEgressMirrorPorts when the switch has enough ports to require the wider bitmask representation. On a high-port-count switch, an admin would set the appropriate bits in this octet string to select egress mirror source ports beyond what the standard PortList mask can represent.