This read-only PerfCounter64 counts the number of delta_ts in the reflector-to-probe direction during which the FLR exceeded the configured availability threshold, per its description. An administrator uses this to quantify bad sub-intervals specifically on the return path, complementing the P2R count to fully localize a loss problem. For example, an operator seeing both P2R and R2P severely-errored counts climbing together would suspect a bidirectional issue like a shared congested link rather than a one-way fault.