An empty element that identifies an instance of the <Subject> composite as representing the primary or main subject category for the product. The main category may be expressed in more than one subject scheme, ie there may be two or more instances of the <Subject> composite, using different schemes, each carrying the <MainSubject/> flag, so long as there is only one main category per scheme (ie per value of <SubjectSchemeIdentifier>). Optional, though for some subject schemes (eg Thema), <MainSubject/> can be vital in order to prevent potential misinterpretation of a group of subject codes, and for other schemes which by their nature only allow one code (eg Dewey), <MainSubject> is not relevant. Non-repeating in each occurrence of the <Subject> composite.