This field is the subnet mask for the cluster synchronization address in haClusterSyncAddr. It completes the addressing picture for the dedicated sync network, which an admin checks alongside haClusterSyncAddr when confirming the sync link is properly isolated and correctly sized for however many members need to exchange state on it. An unusually large mask on what should be a tiny point-to-point sync link between two members might indicate a leftover misconfiguration from a template not tailored to this cluster's size.