MonitoredItemsare entities in the Servercreated by the Clientthat monitor AddressSpace Nodes and, indirectly, their real-world counterparts. When they detect a data change or an event/alarm occurrence, they generate a Notificationthat is transferred to the Clientby a Subscription.

A Subscriptionis an endpoint in the Serverthat publishes Notificationsto Clients. Clientscontrol the rate at which publishing occurs by sending Publish Messages.