This counter tracks the number of ADD PARTY ACKNOWLEDGE messages, confirming that a party was successfully added to a multipoint SVC that this port has received, per the description. Because timer T399 bounds how long the port waits for this specific acknowledgement, comparing this counter to NumAddPtyPdu tells you what fraction of add-party attempts on this port are actually succeeding versus timing out or being rejected. For example, a healthy multipoint service would show this counter tracking NumAddPtyPdu almost one-for-one, with any persistent gap indicating add-party requests that never get confirmed.