This is a 64-bit counter of multicast bytes (octet count) being transmitted from this node onto the fiber, specifically the high-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, SRP's high-priority class is always serviced ahead of low-priority traffic when the ring is congested, so a rising trend here at the expense of the matching low-priority counter is a sign that latency-sensitive traffic is starting to crowd out best-effort traffic on this ring, 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 117,288,938 bytes of multicast traffic over that window, which combined with the matching packet counter lets you compute an average frame size for that class.