This counter tracks the number of DROP PARTY messages, used to remove one participant from a multipoint SVC without tearing down the whole call that this port has received, per the description. This is the normal mechanism for a participant leaving a conference-style ATM call while the connection stays up for everyone else, so its rate reflects churn within multipoint sessions on this port rather than whole-call churn captured by NumReleasePdu. For example, on a conferencing trunk with frequent join/leave activity, this counter would track closely with NumAddPtyPdu over the life of a session.