Yes, I found it:
Code:
Name: title-type
Description:
The title-type property indicates the form or nature of a title.
When the title-type value is drawn from a code list or other formal enumeration, EPUB creators SHOULD attach a scheme attribute to identify its source.
When a scheme is not specified, reading systems SHOULD recognize the following title type values: main, subtitle, short, collection, edition and expanded.
Allowed value(s): xsd:string
Cardinality: zero or one
Extends: dc:title
With example:
Code:
<meta refines="#ID12345" property="prism:contentType">article</meta>
<meta property="dc:title" id="t4main">
Meet The Rejuvenated, Revitalized LeBron
</meta>
<meta refines="#t4main" property="title-type">main</meta>
<meta property="dc:title" id="t4sub">
After a tumultuous first year in Miami, LeBron James locked himself
in his house, rued disappointing his teammates—then worked hard to
hone his game. The result: one of the best seasons in NBA history
</meta>
<meta refines="#t4sub" property="title-type">subtitle</meta>
<meta refines="#ID12345" property="dc:creator">LEE JENKINS</meta>
Hmm, I fear the whole thing would be moot as it is unlikely that any e-reader currently is able to use this. OTOH: it could prepare e-books for the future.