Such an identifier may be created by a publisher or archive, and there is no requirement that identifiers be unique.
Design Note: An element, rather than an attribute, was chosen as the means to capture DOIs and other identifiers so multiple identifiers could be retained.
<!ELEMENT object-id (#PCDATA) >
Text, numbers, or special characters
<abstract> Abstract; <boxed-text> Boxed Text; <chem-struct> Chemical Structure (Display); <chem-struct-wrapper> Chemical Structure Wrapper; <citation> Citation; <fig> Figure; <graphic> Graphic; <media> Media Object; <product> Product Information; <related-article> Related Article Information; <supplementary-material> Supplementary Material; <table-wrap> Table Wrapper