The text of a subject heading taken from the scheme specified in the <SubjectSchemeIdentifier> element, or of free language keywords if the scheme is specified as ‘keywords’; or the text equivalent to the <SubjectCode> value, if both code and text are sent. Either <SubjectCode> or <SubjectHeadingText> or both must be present in each occurrence of the <Subject> composite.

Optional, and repeatable if the heading text is sent in multiple languages. The language attribute is optional for a single instance of <SubjectHeadingText>, but must be included in each instance if <SubjectHeadingText> is repeated.