It’s a bit of a special case. You use a helper function which outputs performance data where the names differ from that of the metric definitions. The metric translation is used to translate the names in the performance data (like e.g. outerr) to the correct metric names (if_out_err in this case).
Usually the names in the performance data and metric definitons are identical. In that case no translation is needed. But for historical reasons it’s different here.