OSO book subdiscipline
Capture the subdiscipline code (sdCode element) and subdiscipline expansion (sdExpansion element) from the Title Passport inside the OSOsubdiscipline element.
bookMeta/OSOsubdiscipline
An OSO book can have a maximum of two subdisciplines.
<bookMeta>
<OSOsubdiscipline><sdCode>ecofin</sdCode><sdExpansion>Financial Economics</sdExpansion><sdCode>ecohis</sdCode><sdExpansion>Economic History</sdExpansion>
</OSOsubdiscipline>
</bookMeta>