Container element to hold the various titles for a structured Index (or a subdivision of such an Index).
The following, in order:
<label> Label of a Figure, Reference, Etc., zero or one
<subtitle> Article Subtitle, zero or more
<trans-title-group> Translated Title Group, zero or more
<alt-title> Alternate Title (in an Index), zero or more
<fn-group> Footnote Group, zero or one