As per definition of subjectset:
"A document can be described using terms from more than one controlled vocabulary. In order to do this, you should use the Scheme attribute to distinguish between controlled vocabularies."
The PACS (Physics and Astronomy Classification Scheme) is a vocabulary of terms used in scholarly publications. It's a table of key values and corresponding descriptions. To properly tag descriptions, subjectterm needs inline math markup (like the term elements have) or eventually subject could have e.g. a subjectdescription element with inline markup.