An OPC UA Subscription groups several MonitoredItems and publishes value changes periodically (PublishingInterval typically 100-1000 ms); the server keeps a buffer of unacknowledged notifications for reliability.

Subscriptions are the optimised push mechanism of OPC UA. A MonitoredItem points at a node attribute and carries its own sampling interval and deadband. A Subscription groups N MonitoredItems and is configured with a PublishingInterval. The client periodically sends PublishRequests and the server returns the accumulated notifications. The KeepAliveCount and LifetimeCount counters handle client inactivity and prevent server-side memory leaks. The result is one network transaction for hundreds or thousands of value changes, far cheaper than individual Reads.

Always size LifetimeCount as at least three times KeepAliveCount, otherwise a transient network glitch will drop the subscription and force a costly resubscribe of every MonitoredItem.