This counts unreliable, no-ack, EIGRP unicast packets sent on this interface, generally packets like Hellos directed at a specific peer rather than broadcast to the segment. On point-to-point links where EIGRP often unicasts rather than multicasts, this counter effectively substitutes for the multicast Hello counter as the interface's basic liveness indicator. A point-to-point serial interface where this counter increases steadily but cEigrpUMcasts stays at zero simply reflects that this link type carries EIGRP traffic unicast rather than multicast -- expected, not a fault.