An ONIX code which identifies the category or subject heading scheme which is used in an occurrence of the <Subject> composite. Mandatory in each occurrence of the composite, and non-repeating.

For category schemes that use code values, use the associated <SubjectCode> element to carry the value (if so required, the <SubjectHeadingText> element can be used simultaneously to carry the text equivalent of the code). For schemes that use text headings, use the <SubjectHeadingText> element to carry the text of the category heading.