Identifier Class Elements
Class comprising the <object-id> element, which is used to capture a publisher’s or archive’s DOI or other identifier.
An element, rather than an attribute, was chosen as the means to capture DOIs and other identifiers so multiple identifiers could be retained. This element may be included at many levels, for example, within an <abstract> or a <fig> element.
<!ENTITY % id.class "object-id" >