This setting controls whether physical layer errors are also remapped (mirrored) along with normal traffic, or reports unsupported(3) if the device is incapable of doing so. Admins would enable this when they need a monitoring port to see not just good frames but also physical-layer error conditions from the source interface. For example, a device that returns unsupported(3) on a get cannot have this value changed, since the hardware doesn't support remapping physical errors.