This counter tracks the number of DROP PARTY ACKNOWLEDGE messages, confirming that a participant was successfully removed from a multipoint SVC that this port has received, per the description. Timer T398 governs how long the port waits for this specific acknowledgement before forcing the removal anyway, so a gap between NumDropPtyPdu and this counter on a busy conferencing port would indicate far-end legs that are slow or failing to confirm party removal. For example, a stable multipoint service should show this counter essentially matching NumDropPtyPduTx one-for-one with little lag.