This read-only PerfCounter64 is the historical counterpart to f3EsaProbeStatsSoamPmExtSumOfSqR2PFDR, recording the sum of squares of backward FDR samples for a completed, archived interval. An administrator's analysis tools use this to compute the historical variance of return-path delay variation for a past interval. For example, an engineer would use this figure to check whether the return path became noticeably less stable in the interval right before a known outage.