A classification scheme is the product of arranging things into kinds of things (classes) or into groups of classes. In the abstract, the resulting structures are a crucial aspect of metadata, often represented as a hierarchical structure and accompanied by descriptive information of the classes or groups. Such a classification scheme is intended to be used for an arrangement or division of individual objects into the classes or groups, and the classes or groups are based on characteristics which the objects (members) have in common. In linguistics, subordinate concepts are described as hyponyms of their respective superordinates; typically, a hyponym is 'a kind of' its superordinate. The ISO/IEC 11179 metadata registry standard uses classification schemes as a way to classify administered items, such as data elements, in a metadata registry. Some quality criteria for classification schemes are: \n* Whether different kinds are grouped together. In other words whether it is a grouping system or a pure classification system. In case of grouping, a subset (subgroup) does not have (inherit) all the characteristics of the superset, which makes that the knowledge and requirements about the superset are not applicable for the members of the subset. \n* Whether the classes have overlaps. \n* Whether subordinates (may) have multiple superordinates. Some classification schemes allow that a kind of thing has more than one superordinate others don't. Multiple supertypes for one subtype implies that the subordinate has the combined characteristics of all its superordinates. This is called multiple inheritance (of characteristics from multiple superordinates to their subordinates). \n* Whether the criteria for belonging to a class or group are well defined. \n* Whether the kinds of relations between the concepts are made explicit and well defined. \n* Whether subtype-supertype relations are distinguished from composition relations (part-whole relations) and from object-role relations.

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