This read-only PerfCounter64 reports the maximum round-trip (two-way) Frame Delay Range, in microseconds, observed during the current measurement interval, per its description, combining both directions into a single end-to-end figure. An administrator uses this when the SLA is defined in round-trip terms rather than per-direction, to catch the worst two-way delay-variation spike. For example, an operator supporting a round-trip-SLA service would flag the interval if f3EsaProbeStatsSoamPmExtMaxRTFDR exceeds the contracted round-trip delay-variation limit.