This read-only PerfCounter64 counts reflector-to-probe unavailable delta_ts, incrementing under the same threshold-crossing rule as its P2R counterpart but for the return direction, per its description. An administrator tracks this to capture SLA-violating downtime specifically on the return path, which might differ from the outbound figure on an asymmetric link. For example, an operator investigating a customer's one-way voice quality complaint would check whether f3EsaProbeStatsSoamPmExtR2PUnavailableTime is elevated relative to the P2R figure.