This counts unicast frames transmitted out this interface, the traffic class that carries real point-to-point service traffic. Because unicast dominates genuine outbound user and service traffic, this is the counter to graph over time for this specific port's transmit side when sizing its traffic pattern or validating a change, rather than the broadcast or multicast counters on the same interface. For example, TxUcastPkts on this port staying flat for this in-progress short-interval bin while RxUcastPkts climbs would show a one-sided conversation worth checking for a routing or forwarding problem on this exact interface.