If your application needs to monitor changes of certain process value (OPC Classic item, OPC UA monitored item), it can subscribe to it, and receive notifications when the value changes. For performance reasons, this approach is preferred over repeatedly reading the item’s value (polling). Note that OPC Data Client has internal optimizations which greatly reduce the negative effects of polling, however subscription is still preferred.

OPC Data Client contains methods that allow you to subscribe to OPC Classic items or OPC UA monitored items, change the subscription parameters, and unsubscribe.