This read-only PerfCounter64 counts reflector-to-probe available delta_ts, incrementing under the same 'n consecutive delta_ts' rule as its P2R counterpart but for the return direction, per its description. An administrator uses it to compute the return-path availability percentage that, together with the P2R figure, forms a complete bidirectional SLA availability picture. For example, an operator would report the lower of the two available-time figures as the binding availability number if the SLA requires both directions to meet the target.