An implementation is not required to issue an Event for every update as it occurs. An OPC UA Server may be capable of grouping a series of transactions or simple updates into a larger unit. This series may constitute a logical grouping or a temporal grouping of changes. A single ModelChangeEvent may be issued after the last change of the series, to cover all of the changes. This is referred to as Event compression. A change in the NodeVersion and the ViewVersion may thus reflect a group of changes and not a single change.