This narrows the CCM transmit counter down to just the CCMs that had to be flooded rather than unicast-switched, which for CCMs, always sent to a multicast MD-level address, is normally the great majority of them. It is useful as a cross-check against the total CCM-sent counter: if the flooded count is noticeably lower, some CCMs are being switched via a learned MAC entry instead of flooded, which is unusual for multicast heartbeat traffic and worth investigating. In a healthy deployment these two counters typically track each other almost 1:1, so any persistent gap between them is the anomaly to chase.