The index-base element allows indexing extensions to be added by specializing this element.
The index-base element can only exist as a child of an indexterm element. This characteristic makes it the appropriate element to specialize to add indexing extensions. For example, the index-see, index-see-also, and index-sort-as elements only make sense as children of indexterm and so are specializations of index-base. Those elements are all part of the indexing domain.
On its own, index-base has no meaning. Processors should ignore this element and its content if encountered in its unspecialized form.
See appendix for information about this element in OASIS document type shells.
The index-see-also element is specialized from index-base; see index-see-also for an example of how index-base can be used with specialization.