Persistent item classes are for example: person, geographical place or group/organisation. All entities of these classes, like for example 'Josef Mayer' as an entity of the class 'person', exist over time. Persistent items are related to temporal entities/events, which describe them. The person 'Josef Mayer' is naked as such. Only through its relation to temporal entities/events is he been given cloths and colors. In the case of 'Josef Mayer', this might be his 'birth' and 'death', which limit his existence.