This octet-string bitmask is the extended-width equivalent of rcMirrorPolicyPorts, selecting which ports are subject to policy-based mirroring, used on switches with more ports than the standard PortList type can address. Admins use it instead of rcMirrorPolicyPorts when the switch's port count requires the wider bitmask representation. On a high-port-count switch, an admin would set the appropriate bits in this octet string to apply the mirror policy to ports beyond what the standard PortList mask can represent.