Name or identifier of an entire journal issue (for example, a DOI).
External Identifier: This element holds an external
identifier, typically assigned to an article by a publisher. The contents of this element should not be confused with the
@id attribute, which holds an internal document identifier that can be used by software to preform a simple link.
The related element <issue> records the published issue number of the journal. This <issue-id> element is usually a more complex number, but a few publishers use the issue number plus a prefix or suffix as the issue