Consistent description of the elements based on defined formal criteria

A wrapper is a container element that is not repeatable and cannot have any attributes other than a language attribute. The description is introduced with "A wrapper ...".
A set is a container element that is (usually) repeatable and can have attributes. The description is introduced with "A set ...".
An index element is an element that can contain values, often as skos: Concept, that can be included in the indexate as such. Data values should be "Controlled". The description is introduced with "An index element ...".
A text element is an element that can contain values. Data values should always be "Free text". The description is introduced with "A text element ...".

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