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A key is an object type or complex data type defined in the context of an owning type (object type or complex data type). A key defines a projection of the owner type by defining one or more attributes as key components.

A key is defined as data type projection from a complex data type, i.e. a key refer to a subset of properties of a complex data type. Keys are complex data types.

Keys are defined in order to identify instances or in order to support search indexes. Keys can be defined as members of complex data types (structures). A key may consist of any number of structure attributes. Since attributes might be transient attributes, which may contain derived values, keys can implicitly defined based on function values.

Keys allow providing instance access mechanisms (locating an instance by key value) and may also act as identifier for an instance.

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