This is a 64-bit counter of multicast bytes (octet count) arriving off the fiber into this node, specifically the low-priority portion of that traffic, per SRP's two-level QoS scheme, measured over the current, still-accumulating 15-minute window (it resets to zero at the next quarter-hour boundary). Because this is measured on the fiber side, it includes traffic this node is simply relaying past on behalf of other ring nodes, not just traffic this router itself originates or consumes, because SRP always drains high-priority traffic first, a plateau or drop in this low-priority counter while the matching high-priority counter is climbing indicates best-effort traffic is being squeezed out by real-time/control traffic rather than genuinely dropping off, and because multicast frames on a shared ring get passed along until they're stripped off or time out, a sudden multicast surge here more often reflects a flooding condition (e.g. an ARP-like storm or a new multicast subscriber group) than ordinary point-to-point application growth, and pairing this byte count with the matching packet counter for the same traffic class is how you'd derive the average frame size and the actual bit rate that class is consuming. For instance, this counter might read roughly 137,225,318 bytes of multicast traffic over that window, which combined with the matching packet counter lets you compute an average frame size for that class.