sort-key Sort Key Version of a Term

Provide an alternative version of, for example, an index term, for use as when sorting or indexing the term. For example, an ASCII version of the index term could replace an accented character with an unaccented one.
OPTIONAL on element: <index-term>
Value Meaning
Text, numbers, or special characters An alternative version of the index entry for sorting.
Restriction @sort-key is an optional attribute; there is no default.