NUAO provides a hierarchy of event classes all derived from nuao:Event. They were designed with usage events triggered by the user in mind.

If looking at the example of a file being opened NUAO models one main event which stretches from the time the file is opened to the time it is closed. This is described via nuao:UsageEvent and the properties nuao:start and nuao:end. During the time span of this event the file can be in the focus of the user or not. This is modelled via nuao:FocusEvent instances that describe when a file was in focus. Since users will typically switch the focus between applications and resources very often storing all focus events might result in a very high number of events that are hard to handle. For this purpose NUAO provides nuao:totalFocusDuration which can be used to accumulate the total amount of time the resource was in focus during a nuao:UsageEvent.

Since the action of modifying a resource is a very foggy concept and hard to fit into an event with start and end time it was decided to instead only store a timestamp for each modification of the resource. This is described via nie:modified and nie:contentModified. For file modifications one would typically use nie:modified.

The diagram above shows one main nuao:UsageEvent which contains three focus events. (The term contains is only used with respect to the visualization. In fact the nuao:FocusEvent instances are only related to the nuao:UsageEvent via their start and end time.) In addition the file was saved three times, twice in the first nuao:FocusEvent and once in the third.