Exactly as described, this counts 256-to-511-byte frames received on this benchmark port. This range corresponds to the "c" (256-byte) EMIX letter, so tracking it alongside the equivalent transmit-side bucket during a mixed-size test verifies mid-sized frames specifically are transiting cleanly, isolating whether any loss reported by the overall test is concentrated in this particular size range. For example, if frame loss during a mixed EMIX test is concentrated in this bucket but not others, that points to a size-dependent issue, such as a QoS policer with a size-based rule, rather than a generic capacity problem.