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.