The /FLAG option indicates whether data with highly deviating weights should be flagged. UV_REWEIGHT DO combines the two actions of UV_REWEIGHT ESTIMATE and UV_REWEIGHT APPLY, but with a control over these outliers (that are not identified by APPLY).
Default Threshold is 3, i.e. data are considered as outliers if their re-scale factor is more than 3 times above (or below) the median value. This is adequate to spot bad data.
/FLAG is only valid for keyword DO.