Keys

A key is an object type defined in the context of an object type. A key consists of a subset of properties defined for the object type owning the key. Thus, a key also defines a projection rule for object instances of the object type. A key consists of one or more key components (property relations). A key component is a feature relation that refers to properties defined for the object type that owns the key. A key may consist of any number of key components, but requires at least one.

At the first glance, keys seem to be an implementation issue, but this is not always case. Since keys are used for identifying object instances or ordering object instances in an object collection, keys have got a conceptual meaning, too.