This object carries a serial number for an alarm occurrence, valid only as a variable binding within a Huawei alarm trap. Admins use it to uniquely distinguish between repeated occurrences of the same alarm type, since two traps with the same hwAlarmId but different hwAlarmSn represent distinct events. For instance, if a link flaps repeatedly, each resulting trap would carry an incrementing hwAlarmSn value even though hwAlarmId stays the same.