This element always holds a subsidiary part of a title, but it has different applications in several contexts:
Within the standards document metadata, a <subtitle> holds one secondary part of one of the standards document component titles: <intro>, <main>, or <compl>.
Some structural components of a standards document (such as Indexes and Tables of Contents) may take also contain <subtitle>s.
Any combination of:
Text, numbers, or special characters
External Linking Elements
<inline-supplementary-material> Inline Supplementary Material
Related Material Elements
Emphasis Elements
Inline Display Elements
Math Elements
Other Inline Elements
Baseline Change Elements
Citation Elements