This read-only Counter64 counts Head-Of-Line drops -- packets discarded because a stalled queue blocked others behind it in the buffer -- broken out per Class-of-Service queue on this specific physical interface. An admin uses the per-CoS breakdown to identify exactly which traffic class is suffering HOL blocking (rather than just seeing one aggregate port-wide drop count), which usually points at that specific queue being undersized or its egress port being oversubscribed.