A Statistical Classification has a structure which is composed of one or several Levels. A Level often is associated with a Concept, which defines it. In a hierarchical classification the Classification Items of each Level but the highest are aggregated to the nearest higher level Level. A linear classification has only one Level.
A Statistical Classification is a subtype of Node Set. The relationship between Statistical ClassifiationClassification and Level can also be extended to include the other Node Set types - Code List and Category Set.