Where does this come from?

You can add proofreader:

<xs:element minOccurs="0" maxOccurs="1" default="" name="Proofreader" type="xs:string"/>

Also, you can add proofreader for translation version:

<xs:element minOccurs="0" maxOccurs="1" default="" name="TranslationProofreader" type="xs:string"/>

What is the rationale for adding support for this element?

A proofreader is a professional who checks written content for errors, ensuring it's polished, accurate, and ready for publication or final use.

Is the element already handled by any application or tool?

No