This counter tracks frames withheld from flooding because a MEP configured at a higher Maintenance Domain level on this device intercepted them first, the CFM level-filtering rule that lets a higher-level MEP shield lower-level traffic from leaking further out. It is a normal, expected counter in any properly nested MD hierarchy -- a provider-level MEP at a UNI legitimately blocks lower-level customer CFM frames from being flooded past it. A steadily incrementing value on a UNI-facing port is exactly what a correctly segmented customer/provider domain looks like; a value stuck at zero on a port that is supposed to have a higher-level MEP would be the anomaly worth investigating.