This sets the near-end loss threshold above which this device will raise a fault for this synthetic-loss session, and per the description a value of -1 disables fault generation entirely while any value 0 or greater is compared directly against the measured near-end loss result. Setting this to 5, for example, would mean a measured near-end loss of 6 or more frames trips a fault, giving the admin an automated alarm rather than needing to manually watch the raw loss counters after every test iteration.